[Core/Realmd] Fix crash when database version doesn't match.

The general gut feeling is that the Runnable shouldn't ref-count itself, but rather eg.
ACE_Strong_Bound_Ptr should be used for that purpose to ensure deletion at the correct
time. This also ensures that we can check if the pointer is available or not, currently that's
not possible at all as we `delete this;` Comments?

Also remove #define MANGOS as that's defined on the command line when building