This is an enhancement change to to cater support all
the data fields present on the object. Currently
we only send a subset of data which object info
provides us.
It also helps us keep a full namespace mirror on
notification targets for efficient query.
This change adds information like host, port and user-agent of the
client whose request triggered an event notification.
E.g, if someone uploads an object to a bucket using mc. If notifications
were configured on that bucket, the host, port and user-agent of mc
would be sent as part of event notification data.
Sample output:
```
"source": {
"host": "127.0.0.1",
"port": "55808",
"userAgent": "Minio (linux; amd64) minio-go/2.0.4 mc ..."
}
```
`principalId` i.e user identity is kept as AccessKey in
accordance with S3 spec.
Additionally responseElements{} are added starting with
`x-amz-request-id` is a hexadecimal of the event time itself in nanosecs.
`x-minio-origin-server` - points to the server generating the event.
Fixes#3556
* 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`
