[session] deprecate `hijack_user` method
The semantic of this method is wicked and lead to very hard violation of sanity.
(multiple transaction using the very same cursors). We deprecated the API to be
able to drop it as soon as possible.
The method was added long ago for some notification trick. It is not needed
since we sent notification on ``postcommit_event``.
(closes #2781782)
[mail] drop hijack_user usage
The transaction is now commited before notification. We can create a new session
directly and drop `hijack_user` usage.
This prepare #2781782