Implement leader election

Implement a leader election algorithm once we have a connection manager (#5). 

https://github.com/apache/zookeeper/blob/trunk/src/java/main/org/apache/zookeeper/server/quorum/FastLeaderElection.java
http://zookeeper.apache.org/doc/r3.4.6/zookeeperInternals.html#sc_leaderElection