Multithreading issues

[janpaul123]

Spawning threads all the time is a bit of an antipattern (you should use a thread pool) so I don't think this should be part of https://github.com/Zaplib/zaplib/milestone/1, but there are some issues with threading that should be addressed at some point:

• test_multithread deadlocks in Chrome after a while (not sure about other browsers).
• In Safari 15.2 test_multithread doesn't work well at all.

There are some deeper issues that might require language changes, and are less urgent, but I'll mention them here anyway. We might want to split this out to a different ticket at some point.

• Threads leak memory since we never deallocate the TLS/shadow stack (see also this issue).