Deprecate addHolder for addHolders.

lot of code was using ::merge() to create a new AbuseFilterVariableHolder
this is now simplified using a single addHolders method.

merge() still exists as its usful as a static function.
addHolder() is deprecated.

Change-Id: Ia4f6a56f642242a04cf2973b74ce44d91fce00eb
Merge "Deprecate addHolder for addHolders."