BUG-6522: create a dedicated extensions map

BUG-6522: create a dedicated extensions map

SourceSpecificContext already calls out to BuildGlobalContext
to lookup up support statements, hence the potential extension
lookup will be slower.

This lookup occurs before we check for type arguments, hence
we want to keep it fast. Therefore we take a snapshot of all
extensions defined in the global context.

Also make StatementDefinitionNamespace a source-local, so the
definitions do not leak outside of the module where they are
defined.

Change-Id: I0c50de89cee5d4297e8ccd0cb61203c3cfe9a2f3
Signed-off-by: Robert Varga <rovarga@cisco.com>