HBASE-11935 ZooKeeper connection storm after queue failover with slave cluster down (Sukumar Maddineni)

Synchronize peer cluster connection attempts to avoid races and rate
limit connections when multiple replication sources try to connect to
the peer cluster. If the peer cluster is down we can get out of control
over time.
HBASE-11935 ZooKeeper connection storm after queue failover with slave cluster down (Lars Hofhansl, Jesse Yates, Andrew Purtell)

Improve thread handling in ReplicationSource where sources may not
be properly terminated