Improve performance of SchemaContext.findModuleByName()

Maintaining an index multimap allows us to heavily optimize lookups,
especially when coming from restconf, which accepts the latest revision
of a particular module.

Change-Id: I615a3c08508111d9389821d24f51cd35c06b5eb1
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "Improve performance of SchemaContext.findModuleByName()"