add/remove peer requests should be routed through master

ReplicationAdmin directly operates over the zookeeper data for replication setup. We should move these operations to be routed through master for two reasons:

• Replication implementation details are exposed to client. We should move most of the replication related classes to hbase-server package.
• Routing the requests through master is the standard practice for all other operations. It allows for decoupling implementation details from the client and code.

Review board: https://reviews.apache.org/r/54730/