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
* Add configuration parameter "format" for db targets and perform
configuration migration.
* Add PostgreSQL `access` format: This causes Minio to append all events
to the configured table. Prefix, suffix and event filters continue
to be supported for this mode too.
* Update documentation for PostgreSQL notification target.
* Add MySQL `access` format: It is very similar to the same format for
PostgreSQL.
* Update MySQL notification documentation.
As a new configuration parameter is added, configuration version is
bumped up from 14 to 15.
The MySQL target's behaviour is identical to the PostgreSQL: rows are
deleted from the MySQL table on delete-object events, and are
created/updated on create/over-write events.
