OVS-DB: Observables for connection status

The patch exposes two observables from the OVS-DB plugin, which
notify the changes to the connection status. The notification
is synchronous with the addition of the connection identifier
to the OVS-DB connections map. Each observables returns the
modified connection instance.

Signed-off-by: Alex Bikfalvi <alex.bikfalvi@midokura.com>