Refactor replica.cc code to support multiple instances

 * Implement explicit API to create/start/stop/destroy remote clients
 * Merge re-connect loop && fiber code for JOIN and SUBSCRIBE
 * Re-use connection between JOIN and SUBSCRIBE
 * Split SUBSCRIBE code from pull_from_replica()
 * Improve logging and unify 'warning_said' logic
 * Fix fiber_cancel() on a replica fiber
 * Move the single instance of struct replica to cluster.h
 * Remove dependency on replica.h(cc) from recovery.h(cc)
 * Use enum replica_state for states instead of const char *