Fix deadlock in vdf_client

[xearl4]

Add missing logic to handle stopped signal in one-weso proofs.

For one-weso proofs, vdf_client launches two threads to do its work. One
repeatedly squares until a given number of iterations, the second waits
until the first is done squaring and then computes a proof.

The squaring-thread properly handles the "stopped" signal and aborts
early, if signaled. That way, it never reaches the targeted iterations.
The 1weso-thread waits until the target iterations are reached, but does
not handle the "stopped" signal. Thus, for stopped iters, it waits
infinitely.

This situation regularly occurs, vdf_client is running for a bluebox
timelord. Whenever the timelord (Python) process restarts, network
communication errors out, killing the squaring thread. But instead of
the while vdf_client exiting (which would lead to the timelord launcher
cleanly restarting a fresh one), the 1weso thread keeps the vdf_client
alive infinitely.

The fix itself is just as it's done for 2weso proofs and elsewhere. The
1weso caller already handles stopped correctly, so 1weso can just
return a default-constructed proof.

[wjblanke]

thanks! florin can u take a look

[fchirica]

thanks!

[github-actions]

'This PR has been flagged as stale due to no activity for over 60
days. It will not be automatically closed, but it has been given
a stale-pr label and should be manually reviewed.'

[xearl4]

Any chance at merging this so that it gets in the upcoming release?

[hoffmang9]

It is not likely to get into today's beta. It may get into 1.3 actual. @wjblanke ?

[wjblanke]

Thanks again