NECONF-524 : Setting the netconf keepalive logic to be more proactive.

NECONF-524 : Setting the netconf keepalive logic to be more proactive.

https://jira.opendaylight.org/browse/NETCONF-524

When a NETCONF device loses connection with the controller in exceptional
conditions such as the management interface going down, underlying TCP
connection would not be closed as there will be no fin packet exchange or
reset. At this point NETCONF state will be set as connected until the
underlying OS determines the connection is stale which can happen after
considerable amount of time. This fix makes the Keepalive packets more
proactive to determine such conditions.This fix also sets the listener
properly which is required for this fix as well.

Change-Id: I781469ae7865e949e9f2c55e53240341f0b10bdd
Signed-off-by: Balaji Varadaraju <bvaradar@luminanetworks.com>