replication: fix appliers pruning

During pruning of appliers some anon replicas might connect
from replicaset_follow called in another fiber. Therefore we need to
prune appliers of anon replicas first and, moreover, prune them one by
one instead of iterating them, as far as any of them might connect
while we are stopping the other one and it will break iteration.

Part of #4586
Closes #4643