raft: reset the leader on entering candidate state

Not resetting a leader causes vote requests to be ignored instead of
rejected which will make voting round to take more time to fail and may
slow down new leader election.

raft/fsm.cc

@@ -161,6 +161,10 @@ void fsm::become_follower(server_id leader) {
 }
 
 void fsm::become_candidate() {
+    // When starting a campain we need to reset current leader otherwise
+    // disruptive server prevention will stall an election if quorum of nodes
+    // start election together since each one will ignore vote requests from others
+    _current_leader = {};
     _state = candidate{};
     _tracker = std::nullopt;
     _log_limiter_semaphore = std::nullopt;