Add test coverage inspired by etcd for non-voter servers,
and fix issues discovered when testing.
* scylla-dev/raft-learner-test-v4:
raft: (testing) test non-voter can vote
raft: (testing) test receiving a confchange in a snapshot
raft: (testing) test voter-non-voter config change loop
raft: (testing) test non-voter doesn't start election on election timeout
raft: (testing) test what happens when a learner gets TimeoutNow
raft: (testing) implement a test for a leader becoming non-voter
raft: style fix
raft: step down as a leader if converted to a non-voter
raft: improve configuration consistency checks
raft: (testing) test that non-voter stays in PIPELINE mode
raft: (testing) always return fsm_debug in create_follower()
7521301b72