BUG-582: Optimize SchemaContextImpl.findModuleByNamespace()

This is by far the hottest method, taking full 10 seconds from the
startup. Optimizing it looks simple enough: just create a SetMultimap
and precompute the URI->Module mappings.

Change-Id: Icf9b42424a083f665911c5b3a8853c38662b1e87
Signed-off-by: Robert Varga <rovarga@cisco.com>