replication: refactor replicaset_leader()

Firstly, rename it to replicaset_find_join_master(). Now, when
there is Raft with a concept of an actual leader, the function
name becomes confusing.

Secondly, do not access ballot member in struct applier in such a
long way - save the ballot pointer on the stack. This is going to
become useful when in one of the next patches the ballot will be
used more.

Part of #5613