BUG-509: merge InMemoryDataTreeModification.resolveSnapshot()

This patch adds some minimal documentation and merges the two
resolveSnapshot() methods into one, making the call stack a bit easier
to understand.

Change-Id: Ib23fccdc9d2987aa255483becc63155904ebc332
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "BUG-509: merge InMemoryDataTreeModification.resolveSnapshot()"