Don't reset election_elapsed if request vote will be rejected.

[BusyJay]

For now a candidate will always reset election_elapsed when receiving a message with higher term. However if the vote is about to be rejected, reset elapsed will make campaign postponed at lease an election timeout.

[wentaoxu]

I think maybe this will make the situation that two nodes start to elect with the same term happen more frequently, and this will make the course of election even longer.