Better asynchronous indexing whilst sticking to single task limit for RPC calls

[TheHolyRoger]

As the title says..

[TheHolyRoger]

@uaktags one thing jumping out at me is that in here:

explorer/lib/database.js

Line 732 in f097289

async.eachLimit(blocks_to_scan, task_limit_blocks, function(block_height, next_block) {

We could build the block list from RPC calls (either in batches or all at once depending on memory), then in a new async loop do all the rest that we're currently doing.

Although a better but more invasive method would be to globally queue + limit RPC calls to a single thread