raft: fsm: when stopping, become follower to reject new requests
raft: fsm: when stopping, become follower to reject new requests

After enabling add_entry forwarding in randomized_nemesis_test, the test
would sometimes hang on _rpc->abort() call due to add_entry messages
from followers which waited on log_limiter_semaphore on the leader
preventing _rpc from finishing the abort; the log_limter_semaphore would
not get unblocked because the part of the server was already stopped.

Prevent log_limiter_semaphore from being waited on when stopping the
server by becoming a follower in fsm::stop.

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