[BUGFIX] TYPO3 Backend is too slow if sys_log table is large

A query is executed during the login process, which can take
minutes during the BE login if the sys_log table is too large.
The query does not use any indexes, operates on the whole table
and uses "filesort". Implementing a proper index solves the
performance issue.

Change-Id: I8f80ed9490d94fb7a318fec687a9ba1ab6e187fe
Fixes: #33532
Releases: 4.7
Reviewed-on: http://review.typo3.org/8771
Reviewed-by: Oliver Klee
Reviewed-by: Stefan Galinski
Tested-by: Stefan Galinski
Reviewed-by: Tolleiv Nietsch
Tested-by: Tolleiv Nietsch
Reviewed-by: Philipp Gampe