Remove subscription on client shutdown

:Release Notes:
Remove subscription on client shutdown instead of
receiving hub notification.

:Detailed Notes:
Previously we were subscribing for a client status
via ls-hub and removing subscriptions after disconnect
notification. This had some limitations. Now we remove
subscriptions on client disconnection using internal
handler.

:Testing Performed:

:QA Notes:

:Issues Addressed:
[PF-688] Luna doesn't always notify when subscriber exits
[BHV-10440] Luna doesn't always notify when subscriber exits
[PF-663] luna-service2: Consider changing _LSTransportMessageGetUniqueToken
         for subscription identification

Open-webOS-DCO-1.0-Signed-off-by: Alexandr Smoktal <alexandr.smoktal@lge.com>

Change-Id: Ie717a98137fe017c507a8cc019855c61d37590ff
Reviewed-on: https://g2g.palm.com/6193
Reviewed-by: DCO Verification
Reviewed-by: Alexandr Smoktal <alexandr.smoktal@lge.com>
Tested-by: Alexandr Smoktal <alexandr.smoktal@lge.com>
Reviewed-by: Anatolii Sakhnik <anatolii.sakhnik@lge.com>