raft: Fix the bug with not setting the current leader.
raft: Fix the bug with not setting the current leader.

When AppendEntries/InstallSnapshot term is the same as the current
server's, and the current servers' leader is not set, we should
assign it to avoid starting election if the current leader becomes idle.

Restructure the code accordingly - change candidate state to Follower
upon InstallSnapshot.

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla