Refactor connection pool

[syeopite]

Brings improvements from https://github.com/jgaskins/http_client which is a shard that implements a HTTP connection pool through DB::Pool like Invidious.

The most noticeable change is that connection timeout errors will no longer say DB::PoolTimeout but rather Invidious::ConnectionPool::Error. Full backtrace here.

Backtrace

Title: `Could not check out a connection in 5.0 seconds (Invidious::ConnectionPool::Error)`
Date: `2024-11-12T00:15:28Z`
Route: `/search?q=test`
Version: `2024.11.11-c026a7b1 @ connection-pool-improvements`

<details>
<summary>Backtrace</summary>
<p>
   
```
Could not check out a connection in 5.0 seconds (Invidious::ConnectionPool::Error)
  from src/invidious/connection/pool.cr:45:7 in '_post_json'
  from src/invidious/yt_backend/youtube_api.cr:569:12 in 'search'
  from src/invidious/search/processors.cr:10:7 in 'regular'
  from src/invidious/search/query.cr:127:9 in 'process'
  from src/invidious/routes/search.cr:61:17 in 'search'
  from src/invidious/routing.cr:184:20 in '->'
  from lib/kemal/src/kemal/route.cr:12:26 in '->'
  from src/invidious/helpers/handlers.cr:30:37 in 'process_request'
  from lib/kemal/src/kemal/route_handler.cr:17:7 in 'call'
  from /usr/share/crystal/src/http/server/handler.cr:30:7 in 'call_next'
  from lib/kemal/src/kemal/websocket_handler.cr:13:14 in 'call'
  from /usr/share/crystal/src/http/server/handler.cr:30:7 in 'call_next'
  from lib/kemal/src/kemal/filter_handler.cr:21:7 in 'call'
  from /usr/share/crystal/src/http/server/handler.cr:30:7 in 'call_next'
  from src/invidious/helpers/handlers.cr:157:5 in 'call'
  from /usr/share/crystal/src/http/server/handler.cr:30:7 in 'call_next'
  from src/invidious/helpers/handlers.cr:94:12 in 'call'
  from /usr/share/crystal/src/http/server/handler.cr:30:7 in 'call_next'
  from src/invidious/helpers/handlers.cr:146:5 in 'call'
  from /usr/share/crystal/src/http/server/handler.cr:30:7 in 'call_next'
  from src/invidious/helpers/handlers.cr:70:5 in 'call'
  from /usr/share/crystal/src/http/server/handler.cr:30:7 in 'call_next'
  from src/ext/kemal_static_file_handler.cr:162:16 in 'call'
  from /usr/share/crystal/src/http/server/handler.cr:30:7 in 'call_next'
  from lib/kemal/src/kemal/exception_handler.cr:8:7 in 'call'
  from /usr/share/crystal/src/http/server/handler.cr:30:7 in 'call_next'
  from src/invidious/helpers/logger.cr:21:35 in 'call'
  from /usr/share/crystal/src/http/server/handler.cr:30:7 in 'call_next'
  from lib/kemal/src/kemal/init_handler.cr:12:7 in 'call'
  from /usr/share/crystal/src/http/server/request_processor.cr:51:11 in 'process'
  from /usr/share/crystal/src/http/server.cr:521:5 in 'handle_client'
  from /usr/share/crystal/src/http/server.cr:451:5 in '->'
  from /usr/share/crystal/src/fiber.cr:143:11 in 'run'
  from /usr/share/crystal/src/fiber.cr:95:34 in '->'
  from ???
Caused by: Could not check out a connection in 5.0 seconds (DB::PoolTimeout)
  from lib/db/src/db/pool.cr:261:7 in 'wait_for_available'
  from lib/db/src/db/pool.cr:127:35 in 'checkout'
  from lib/db/src/db/pool.cr:152:20 in '_post_json'
  from src/invidious/yt_backend/youtube_api.cr:569:12 in 'search'
  from src/invidious/search/processors.cr:10:7 in 'regular'
  from src/invidious/search/query.cr:127:9 in 'process'
  from src/invidious/routes/search.cr:61:17 in 'search'
  from src/invidious/routing.cr:184:20 in '->'
  from lib/kemal/src/kemal/route.cr:12:26 in '->'
  from src/invidious/helpers/handlers.cr:30:37 in 'process_request'
  from lib/kemal/src/kemal/route_handler.cr:17:7 in 'call'
  from /usr/share/crystal/src/http/server/handler.cr:30:7 in 'call_next'
  from lib/kemal/src/kemal/websocket_handler.cr:13:14 in 'call'
  from /usr/share/crystal/src/http/server/handler.cr:30:7 in 'call_next'
  from lib/kemal/src/kemal/filter_handler.cr:21:7 in 'call'
  from /usr/share/crystal/src/http/server/handler.cr:30:7 in 'call_next'
  from src/invidious/helpers/handlers.cr:157:5 in 'call'
  from /usr/share/crystal/src/http/server/handler.cr:30:7 in 'call_next'
  from src/invidious/helpers/handlers.cr:94:12 in 'call'
  from /usr/share/crystal/src/http/server/handler.cr:30:7 in 'call_next'
  from src/invidious/helpers/handlers.cr:146:5 in 'call'
  from /usr/share/crystal/src/http/server/handler.cr:30:7 in 'call_next'
  from src/invidious/helpers/handlers.cr:70:5 in 'call'
  from /usr/share/crystal/src/http/server/handler.cr:30:7 in 'call_next'
  from src/ext/kemal_static_file_handler.cr:162:16 in 'call'
  from /usr/share/crystal/src/http/server/handler.cr:30:7 in 'call_next'
  from lib/kemal/src/kemal/exception_handler.cr:8:7 in 'call'
  from /usr/share/crystal/src/http/server/handler.cr:30:7 in 'call_next'
  from src/invidious/helpers/logger.cr:21:35 in 'call'
  from /usr/share/crystal/src/http/server/handler.cr:30:7 in 'call_next'
  from lib/kemal/src/kemal/init_handler.cr:12:7 in 'call'
  from /usr/share/crystal/src/http/server/request_processor.cr:51:11 in 'process'
  from /usr/share/crystal/src/http/server.cr:521:5 in 'handle_client'
  from /usr/share/crystal/src/http/server.cr:451:5 in '->'
  from /usr/share/crystal/src/fiber.cr:143:11 in 'run'
  from /usr/share/crystal/src/fiber.cr:95:34 in '->'
  from ???
```
</p>
</details>

I've also added a configuration option to customize max idle pool size and the checkout timeout

[syeopite]

TODO: Add connection pool stats to /api/v1/stats

https://crystal-lang.github.io/crystal-db/api/0.13.1/DB/Pool.html#stats-instance-method

https://github.com/crystal-lang/crystal-db/blob/892c25675634fcdf59780055c872be8468b2c204/src/db/pool.cr#L103-L110