Make sure leadership transfer is set back to false in all cases. (#493)

Release 1.3.4 introduced a bug that wound only call setLeadershipTransferInProgress back to false
when leadership transfer to another node succeeded.  In the case where it did not succeed,
leadershipTransferInProgress was left set to true, breaking any callers that relied on it being set back
to false after a failed leadership transfer.

raft.go

@@ -581,10 +581,6 @@ func (r *Raft) leaderLoop() {
 	// based on the current config value.
 	lease := time.After(r.config().LeaderLeaseTimeout)
 
-	// This would unset leadershipTransferInProgress
-	// in case it was set during the loop
-	defer func() { r.setLeadershipTransferInProgress(false) }()
-
 	for r.getState() == Leader {
 		select {
 		case rpc := <-r.rpcCh:
@@ -616,7 +612,18 @@ func (r *Raft) leaderLoop() {
 			// in case eg the timer expires.
 			// The leadershipTransfer function is controlled with
 			// the stopCh and doneCh.
+			// No matter how this exits, have this function set
+			// leadership transfer to false before we return
+			//
+			// Note that this leaves a window where callers of
+			// LeadershipTransfer() and LeadershipTransferToServer()
+			// may start executing after they get their future but before
+			// this routine has set leadershipTransferInProgress back to false.
+			// It may be safe to modify things such that setLeadershipTransferInProgress
+			// is set to false before calling future.Respond, but that still needs
+			// to be tested and this situation mirrors what callers already had to deal with.
 			go func() {
+				defer r.setLeadershipTransferInProgress(false)
 				select {
 				case <-time.After(r.config().ElectionTimeout):
 					close(stopCh)
@@ -853,7 +860,6 @@ func (r *Raft) verifyLeader(v *verifyFuture) {
 
 // leadershipTransfer is doing the heavy lifting for the leadership transfer.
 func (r *Raft) leadershipTransfer(id ServerID, address ServerAddress, repl *followerReplication, stopCh chan struct{}, doneCh chan error) {
-
 	// make sure we are not already stopped
 	select {
 	case <-stopCh: