worker: ability to disable thread unsafe tasks The worker on Windows is implemented using a thread pool. If worker tasks are not thread safe, badness can occur. In addition, if tasks are executing CPU bound code and holding onto the GIL, there will be non-substantial overhead in Python context switching between active threads. This can result in significant slowdowns of tasks. This commit teaches the code for determining whether to use a worker to take thread safety into account. Effectively, thread unsafe tasks don't use the thread-based worker on Windows. Differential Revision: https://phab.mercurial-scm.org/D3962