BUG-6522: streamline QNameToStatementDefinitionMap API

BUG-6522: streamline QNameToStatementDefinitionMap API

This really is a Map, but let's not as far as declaring
it as such.

Add putAll() and putIfAbsent(), which make life way easier
for SourceSpecificContext -- which is the sole user. The end
result is better performance at least in the bulk case,
as we end up populating one of the maps via addAll().

Also take time to formulate the class in terms of lambas,
as that takes a lot of verbosity out.

Change-Id: Ie4f70cbc01011eb49b330ea4411d8308c401f76d
Signed-off-by: Robert Varga <rovarga@cisco.com>