Split out ModificationApplyOperation.verifyStructure() into two methods

This method is heavily overloaded, with a total of 8 implementations, most of which are doing the same thing. Furthermore the original call sites use a static true/false flag, which is indicative the two calls really are separate, with verifyChildren=true including the case of verifyChildren=false.

Further confusion arises from recursivelyVerifyStructure(), which seems to be an unconditional variant of verifyStructure(verifyChildren=true).