threadpool: Eliminate pointless AO2 usage.

thread_worker_pair, set_size_data and task_pushed_data structures are
allocated with AO2 objects, passed to a taskprocessor, then released.
They never have multiple owners or use locking so AO2 only adds
overhead.

Change-Id: I2204d2615d9d952670fcb48e0a9c0dd1a6ba5036