workerpool: Additional check in BaseWorker.ShouldTerminate

Document that it should only be called from within RunTask and
add an assertion for this. This means we can no longer use a
method on the pool and hence remove WorkerPool.ShouldWorkerTerminate.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>