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
(cherry picked from commit e9518fdc4410b5407318d4b3bcc7fa333e296c55)
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>