## Motivation
The initial version of `tokio-compat`'s compatibility runtime added in
#1663 doesn't support the calls to `tokio_threadpool` 0.1's `blocking`.
This is because (unlike the timer, executor, and reactor), there's no
way to override the global `blocking` functionality in
`tokio-threadpool`.
## Solution
As discussed [here][1], this branch adds APIs to the v0.1.x version of
`tokio-threadpool` that allow overriding the behavior used by calls to
`blocking`. The threadpool crate now exposes `blocking::set_default` and
`blocking::with_default` functions, like `executor`, `timer`, and
`reactor`. This will allow `tokio-compat` to override calls to 0.1's
`blocking` to use the new `tokio` 0.2 blocking APIs.
Unlike the similar APIs in `executor`, `timer`, and `reactor`, the hooks
for overriding blocking behaviour are `#[doc(hidden)]` and have comments
warning against their use outside of `tokio-compat`. In general, there
probably won't be a compelling reason to override these outside of the
compatibility layer.
Refs: #1722
[1]: https://github.com/tokio-rs/tokio/pull/1663#issuecomment-548661766
Signed-off-by: Eliza Weisman <[email protected]>
