mink: Race condition for local object processing

There is a race condition in local object processing. When inbound req
is handed over to server thread, an issue could arise if invoke client
is killed just before server pick up request for marshalling. cb_txn
is refcounted but not local objs inside cb_txn. So it causes issue in
marshaling as killing of invoke client releases local obj.

Change-Id: I6ee101f210ea57358b329aeca972f4c6976b6255
Signed-off-by: Dinesh K Garg <dineshg@codeaurora.org>