Close device's source resolution on teardown

Close device's source resolution on teardown

Checking the current connection status is not enough to guarantee that
schema setup was completed for the initial session-up event.

To resolve this close ListenableFuture for device schema resolution
when the 'onRemoteSessionDown' method is called.

This prevents unnecessary duplicate notification, creating duplicate
SchemaContext and avoid confusion in the connection state.

JIRA: NETCONF-1173
Change-Id: I7f84da4a9197db52a0c33d2640e0c2543e587d7b
Signed-off-by: Sangwook Ha <sangwook.ha@verizon.com>
Signed-off-by: Peter Suna <peter.suna@pantheon.tech>
Signed-off-by: Ivan Hrasko <ivan.hrasko@pantheon.tech>