flow.async: Utility to block signals in thread; possibly throw in a convenience API for ..._task_loop.

[ygoldfeld]

~First thing in a given thread started by flow.async (..._task_loop interfaces/classes), it is desirable to block some set of signals, or all signals.

• It is already a feature of ..._task_loop to provide a function to execute first-thing in every started thread in a thread pool. So one can do this manually.
• Would be nice to provide a free function to do this conveniently.
• After that, might be nice to provide some kind of hook to do this in ..._task_loop even more easily than supplying the thing from the preceding point to ..._task_loop::start().