[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
\.svn
^build$
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\.pyc$
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\~$
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\.swp$
^doc/book/en/apidoc$
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syntax: regexp
.*/data/database/.*\.sqlite
.*/data/database/.*\.config
.*/data/database/tmpdb.*
.*/data/ldapdb/.*
^doc/html/
^doc/doctrees/
^doc/book/en/devweb/js_api/