Revision 1379002

SOLR-3750: On session expiration, we should explicitly wait some time before running the leader sync process so that we are sure every node participates.
  
SOLR-3772: On cluster startup, we should wait until we see all registered replicas before running the leader process or if they all do not come up, N amount of time. 

SOLR-3752: When a leader goes down, have the Overseer clear the leader state in cluster.json 

SOLR-3751: Add defensive checks for SolrCloud updates and requests that ensure the local state matches what we can tell the request expected.