Deprecate ORMTable::getFieldPrefix

Deprecate ORMTable::getFieldPrefix

Internal calls to this method show up in my XDebug profiling as
the second most called method (after array_key_exists). Called
10000 times. Sure, it's very cheap and clearly not a bottleneck.
But just not necesarry to have a method for that. The benefit of
having a method is to have a dynamic prefix that can change
depending on other things in an object. But I think this is not
a good idea for a prefix.

Since this is protected it is part of the contract of this class.
I already cleaned known subclasses. Should be @deprecated for
some time and can be removed later.

Change-Id: I46a0d7e072d0a69e9aef5f77e92044b62e3d0ce7