Restrict MD-SAL data operations to a single thread

MD-SAL does not give us transaction ordering guarantees, which mean that
data may hit the data store than what the application intended. Change
the model to single-threaded until proper transaction ordering APIs are
defined.

Change-Id: I6c6375480082b94c614ab2602a2857e0f4779ee8
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "Restrict MD-SAL data operations to a single thread"