Code cleanup FDImpl
Make FD use a Timer instance for it's scan task.
Make FD core use java.util.concurrent
Clean up concurrency code in MembershipImpl
Convert FD over to using cluster size rather than a majority.
Make sure user supplies a cluster size when creating Paxos processes.
Dump the synchronisation around dispatchers for a java.util.concurrent structure
Minimise non-final state in Core
Clean some cruft out of Leader
Reduce visible scope of RecoveryTrigger
Pull the common failure detector validation code into a separate util.