Fix channel inactive handling for SSH transport

Fix channel inactive handling for SSH transport

Channel inactive handler require to re-transfer the event
down the pipeline in order for subsequent handler to be
triggered as well. SSH transport pipeline did contain two
channel inactive handlers both terminating the event
resulting *NetconfSession not handling channel termination
properly.

Fixed, case covered by test.

JIRA: NETCONF-1200
Change-Id: I2a78709909e1b8509b562a8fa0bf640c45c91432
Signed-off-by: Ruslan Kashapov <ruslan.kashapov@pantheon.tech>
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>