Don't close socket while re-initializing notify-listeners, as the rpc
client object is shared between notify-listeners and peer clients.
Also, improves SendRPC() readability by using GetPeerClient().
Fixes a serialisation bug - encoding/gob does not directly support
serializing `map[string]interface{}`, so we serialise to JSON and send a
byte array in the RPC call, and deserialize and update on the receiver.
* Implements a Peer RPC router that sends info to all Minio servers in the cluster.
* Bucket notifications are propagated to all nodes via this RPC router.
* Bucket listener configuration is persisted to separate object layer
file (`listener.json`) and peer RPCs are used to communicate changes
throughout the cluster.
* When events are generated, RPC calls to send them to other servers
where bucket listeners may be connected is implemented.
* Some bucket notification tests are now disabled as they cannot work in
the new design.
* Minor fix in `funcFromPC` to use `path.Join`
