Feature: More granular support for types of blocking futures - frequently blocking vs sometimes, long poll vs briefly blocking poll

[jlizen]

I suspect that the optimal handling strategy for compute heavy futures will be bimodal:

• very likely to block, and/or block for a long time
• occasionally blocking or briefly blocking

For one, it might make more sense to send to a threadpool. The other, maybe something along the lines of block in place or other custom strategies.

It would be good to have an expressive enough API to allow library authors to specify which is which, in case the caller wants to configure strategies separately.

[jlizen]

This is no longer quite accurate, but it'd be nice to have in a sync context as well (enabling block-in-place)