Mesh: Handle Multi-application init & deinit logic

Mesh: Handle Multi-application init & deinit logic

This patch handles following:-
- Introduces Network Unload API. This API
  will be used by application to unload
  all network resources from bt-service.
  During mesh application deinit, FRWK
  library will call Network Unload to cleanup
  resources from service layer. CDB will remain
  unaffected due to this.

- Implements init/deinit logic for multiple apps.
  bt-service keeps reference count of applications.
  mesh stack launch/unlaunch is done single time
  based on app reference count logic. Don't allow
  to stop mesh stack service, if atleast one app
  is using mesh service.

Change-Id: I705c9c4d462036a657184dbdbb131c77af83b3a3
Signed-off-by: Anupam Roy <anupam.r@samsung.com>