flow.async: In ..._task_loop, need a way to supply thread attributes at instantiation (minimally, stack size).

[ygoldfeld]

@ygoldfeld quoting colleague who filed this pre-open-source (I have not contemplated it):

Thread attributes:
Need a way to specify thread attributes for the worker pool threads using boost::thread::attributes
A server could e.g. create approximately 3000 threads for disk I/O, so we want to keep the thread stack sizes small (128KB). At the moment the only requirement is to set worker thread stack size.
Naming the thread looks like it has already been taken care of.

@ygoldfeld adds: See also #61 -- is it related/possible to do similarly?