Re-enable multiple db table prefixes

[simongroenewolt]

Force initialization of ObjectManager for correct spl_object_hash() behavior.
Needed to get the correct spl_object_hash in case it is a proxy

Still needed: revert some changes that were done as workaround for the incorrect behavior of spl_object_hash()

[simongroenewolt]

@bobdenotter I just realized that without talking about it this PR will just be forgotten :-)

Are you interested in getting this merged? I've still marked it WIP as the change itself doesn't do anything, it merely makes it possible to revert your old workaround that made it impossible to have multiple table prefixes. I'm a bit wary to include a revert of those changes in this PR, but maybe that would be the correct thing to do?

[bobdenotter]

Are you interested in getting this merged?

Yes, certainly. What I've done there was undeniably a hack. If we can revert it, I'm all for it. :-)

[simongroenewolt]

Now this PR actually re-enbles the use of different table prefixes -- I've reverted the workaround. As I didn't have time to actually test it I'll mark the PR 'work in progress' again.