Update `SESSION_USER()` behaviour to be comparable with `CURRENT_USER()`.
`SESSION_USER()` will return the user and host columns from `mysql.user`
used to authenticate the user when the session was created.
Historically `SESSION_USER()` was an alias of `USER()` function. The
main difference with `USER()` behaviour after this changes is that
`SESSION_USER()` now returns the host column from `mysql.user` instead of
the client host or ip.
NOTE: `SESSION_USER_IS_USER` old mode is added to make the change
backward compatible.
All new code of the whole pull request, including one or several files
that are either new files or modified ones, are contributed under the
BSD-new license. I am contributing on behalf of my employer
Amazon Web Services, Inc.
remove old deprecation helpers that were not used anywhere.
create new deprecation helpers and enforce their usage
this also removes inconsistencies in reporting deprecation:
sometimes it was ER_WARN_DEPRECATED_SYNTAX (1287),
sometimes ER_WARN_DEPRECATED_SYNTAX_NO_REPLACEMENT (1681),
sometimes a warning, sometimes a note.
it should always be
* ER_WARN_DEPRECATED_SYNTAX
* a warning (because it's something actionable, not purely informational)