Introduce higher-level "network client" APIs

[armanbilge]

FS2's Network APIs are frequently used to build clients such as Ember (HTTP client) and Skunk (Postgres client). These implementations build on top of the "low-level" Socket APIs and introduce higher-level concepts such as persistent, pooled connections. Currently there is both duplicated code and duplicated (and also unique!) bugs across Ember and Skunk and probably other client libraries in our ecosystem.

• switch to keypool? skunk#266
• Fix cancelation when waiting on pool skunk#919
• Deadlock while using session pool skunk#1042
• Fix cancelation of taking from pool keypool#518
• Broken Pipe Exception from Pool skunk#524
• Detect terminated EmberConnections in ember client prior to attempting a request http4s/http4s#6980
• MaxTotal does not take idle resources into account keypool#440
• Ember client hangs when remote server idle timeout is shorter than client's http4s/http4s#7216

I propose that we introduce to FS2 a new set of high-level APIs that formalize "Connection" and help handle its lifecycle within a configurable pool. The goal would be that this abstraction is something that Ember, Skunk, and other client libraries could build on top of.

[mpilquist]

I created a temporary repository to experiment with the API here: https://github.com/typelevel/fs2-cnxs

If successful, we'd move everything directly in to fs2-io.