BUG-372: Rework sal-netconf-connector

This makes the connector truly asynchronous and able to work with
backing datastore going away. It will retry connecting infintely to the
backed device, too.

Change-Id: I843620bf63eeade231698c22592aaec0521a09ad
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "BUG-372: Rework sal-netconf-connector"