[Feature request] Add sharding by specified key

[Alexfilus]

Last 7 years I only met projects where ACID transactions were important only for scope of user.
So I am using multiple PostgreSQL clusters sharded on client by userID.
It would be nice to have such feature in yours library.
For example user can specify sharding function on init. And pass sharding key to connection manager before performing request.

WithShardingFunction(func(key int) { return key % 10})

WithShards(shards map[int][]*Node) something like that