Add `cluster/storeFile` cluster event

I saw that `cluster/deleteFile` is called in `eZDFSFileHandlerMySQLiBackend::_store()` and thought this couldn't be the intended use.

Then I thought it could be a good idea to introduce a new cluster event that notifies listeners on file creation, as a complement to `cluster/deleteFile`.

So here it is :).

Cheers
:octocat: