BUG-981: RuntimeGeneratedMappingServiceImpl thread safety

This patch improves and documents thread safety of waiting for schema to
become available. There was an obvious race condition between schema
context change and registering for notification.

Change-Id: Ieb3999b3d6af5fbbee63015c92a87353a71229d7
Signed-off-by: Robert Varga <rovarga@cisco.com>