Review Worker code again

[vmx]

As part of my work of moving FFT and Multiexp into the ec-gpu project, I was reviewing the Worker code again.

The current Worker::scoped() code seems to do what the comment says. Though it seems to needlessly use a message passing primitive.

THREAD_POOL.scoped(), where THREAD_POOL is a yastl pool, hence it's call to Pool::scoped(), which according to it's docs is already waiting for the execution to finish before it returns. This means the additional message passing wouldn't be needed.

So either change the call to the pool to Pool::spawn(), which wouldn't wait for the execution to finish, or we remove the message passing. As we want it to block, I suggest we keep relying on Pool::scope().