Extract `pool` implementation to dedicated shard

[Blacksmoke16]

Extracted from https://forum.crystal-lang.org/t/http-client-and-fibers/6068/5, it feels a bit strange that the db shard has a quite robust pool implementation, but in order to use it you have to require a shard that you may otherwise have no reason to use. It would make more sense if the pool was broken out into its own shard, and then required by db.

I did a quick look and think the main challenge would be dealing with the exception types. db would have to rescue/re-raise its own errors, or we make it a minor breaking change and just tell people to update the rescues to the new pool implementation types.

[jgaskins]

Previous discussion of this is in #122.