Separate out ModificationApplyOperation.verifyStructure()

Separate out ModificationApplyOperation.verifyStructure()

The boolean handling of this method makes it really two distinct
methods, quickVerifyStructure() and fullVerifyStructure().

Refactor them so that we have two separate invocation paths, which
meet in SchemaAwareApplyOperation and from there are properly
dispatched to subclasses.

This reduces the number of implementations and makes it much clearer
as to what the specific and shared logic is. Most notably it makes
fullVerifyStructure() have only three implementations and
quickVerifyStructure() becomes bimorphic.

JIRA: YANGTOOLS-947
Change-Id: Ife836c7b9011e97a421746efa575148b1e77f6e9
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>