Delete a replicaset from bucket map when this replicaset is removed from the cluster

[nurzhan-saktaganov]

Original: KaymeKaydex/go-vshard-router#59

In RemoveReplicaset we remove replicaset from idToReplicaset map and close its connection pool.
But in routeMap we may still have pointers to this replicaset. This looks like a buggy behavior.

The straightforward solution with replacing to nil all elements of routeMap, that hold this pointer, doesn't work.
Because there might be a discovering goroutine that holds an old value of idToReplicaset that is still in progress.
Therefore, we need a bit more smart solution.