Improve performance of ObjectTypeCache::init()

Improve performance of ObjectTypeCache::init()

This method was one of the slowest that get’s regularly called
during a request.

According to XDebug profiler a request in an empty, stock setup of
Burning Board spent 7% of it's time in this method when requesting
?board-list/. 14% if you are including methods called by this method.

While that obviously does not represent a real world community
optimizing this method is sound, as it’s easily done:

After moving the θ(n) calculation that groups object types by their
definition into the cache where it belongs the numbers go down to
0.31% (excl) and 1.80% (incl).