r/hb_mgr: check if current replica is a leader when collecting hbeats

Some time ago we made collecting heartbeats an asynchronous process
(added yields in between creating hb requests for individual nodes).
This made it possible for leadership to be already lost while the
request for group is created as the yield happened and previously
validated condition no longer holds. Trying to suppress the heartbeat
while the replica is not leader lead to assertion.

Added a check to make sure the leadership is not lost when actually
creating a heartbeat request for the follower.

Signed-off-by: Michał Maślanka <michal@redpanda.com>

src/v/raft/heartbeat_manager.cc

@@ -82,6 +82,12 @@ heartbeat_manager::requests_for_range() {
           r->_fstats.end(),
           [this, r, last_heartbeat, &pending_beats, &reconnect_nodes](
             follower_stats::value_type& p) {
+              // we need to check again as the leadership might have been lost
+              // while we yield.
+              if (!r->is_elected_leader()) {
+                  return;
+              }
+
               auto& [id, follower_metadata] = p;
               if (
                 follower_metadata.last_received_reply_timestamp