Rename Trigger -> Event to be a more appropriate
name for the audit event.
Bonus: fixes a bug in AddMRFWorker() it did not
cancel the waitgroup, leading to waitgroup leaks.
time.Format() is not necessary prematurely for JSON
marshalling, since JSON marshalling indeed defaults
to RFC3339Nano.
This also ensures the 'time' is remembered until its
logged and it is the same time when the 'caller'
invoked 'log' functions.
This is to ensure that there are no projects
that try to import `minio/minio/pkg` into
their own repo. Any such common packages should
go to `https://github.com/minio/pkg`
