ssa.c: Block access thread until all pushed tasks have been processed

Glib implementation is not clear what exactly
g_thread_pool_unprocessed returns: number of waiting tasks
or number of waiting + processing tasks.
Instead of g_thread_pool_unprocessed, access will use an
atomic variable: num_tasks.

Signed-off-by: Sasha Kotchubievsky <sashakot@mellanox.com>
Signed-off-by: Hal Rosenstock <hal@mellanox.com>