(Bully algorithm) Don't start a new election if there is another in progress

[HRezaei]

https://github.com/Hrezaei/Leader-Election-With-Nanomsg/blob/master/bully.cpp#L173

According to the description of bully election in the page 645 of the book "Distributed systems concepts and design 5th edition" (George Coulouris), when a process p[i] receives an election message from p[j], p[i] sends an answer message to p[j] and starts a new election, "unless it has begun one already"