This change adds `access` format support for notifications to a Redis
server, and it refactors `namespace` format support.
In the case of `access` format, a list is used to store Minio
operations in Redis. Each entry in the list is a JSON encoded list of
two items - the first is the Minio server timestamp of the event, and
the second is an object describing the operation that created/replaced
the object in the server.
In the case of `namespace` format, a hash is used. Entries in the hash
may be updated or removed if objects in Minio are updated or deleted
respectively. The field values in the Redis hash are JSON encoded.
Also updates documentation on Redis notification target usage.
Towards resolving #3928
Aditya Manthramurthy
61b08137b0