ssa.c: Add thread termination and cleanup mechanism for prdb_handler

When access service is stopped, cancel signal is sent
to prdb update queue handler. As a consequence of that
cleanup handler, that is cleaning the update queue,
prdb_handler_cleanup is called and after that the thread
halts its execution.

Signed-off-by: Ilya Nelkenbaum <ilyan@mellanox.com>
Signed-off-by: Hal Rosenstock <hal@mellanox.com>