Introduce a new base class for replication peer procedure

As the sync replication peer state transition will have more steps than normal replication peer, it will be good to have a common base class for them.

Since the peer id will be stored in this class, I tend to change the protobuf message name from 'ModifyPeerStateData' to 'ReplicationPeerProcedureStateData'. This will be committed to master and HBASE-19397-branch-2.