HBASE-11963 Synchronize peer cluster replication connection attempts (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-11964 Improve replication source thread handling (Lars Hofhansl, Jesse Yates, Andrew Purtell)

Improve replication source thread handling. Limit parallelism when
transferring queues. Ensure replication sources terminate properly.