Make the ACP session access log more useful

Make the ACP session access log more useful

This commit intentionally overloads the existing fields to keep the change
simple. The log is not really usuable for programmatic processing, due to the
existing constraints. The cleanest solution probably would be storing all the
loggable information as a JSON blob and then parsing that one when accessing
the log and querying it with MySQL's JSON function. But the latter are not
supported yet.

All in all this change improves the ACP session access log a lot and that is
what counts.

Resolves #3537