Client connection pool library

[fireduck64]

Need to figure out where exactly to put this. Maybe a new form of stub? It should automatically connect to some nodes, get peer info and maintain connections to nodes to cover all current shards.

Ideally we would also have a layer to do some operations without regard to what shards, like get utxos across all shards. Maybe that should be on top of the connection pool layer. Not sure yet.