BUG-509: use dedicated executor for notifications

This dedicates a thread to datastore notifications, such that they are
delivered outside of the invoking threads. Also adds proper headers.

Change-Id: I44aeb9c4fefb8784d22ded9ae591d38993905ffa
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "BUG-509: use dedicated executor for notifications"