BUG-1275: optimize AugmentationSchemaProxy for lookup speed

This patch uses ImmutableMap to bias the map from creation speed to
lookup performance. Also, in the lookup path we perform a single lookup,
and check the null result, instead of contains() followed by a lookup.

Change-Id: Iba50dc5fae7ef87da79072be4936690587f46018
Signed-off-by: Robert Varga <rovarga@cisco.com>