This is a consolidation effort, avoiding usage
of naked strings in codebase. Whenever possible
use constants which can be repurposed elsewhere.
This also fixes `goconst ./...` reported issues.
* 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`
