Make ThreadPool optional

[taiki-e]

This makes ThreadPool optional (disabled by default).
cc #1893

Closes #1724

r? @cramertj
cc @seanmonstar

[Matthias247]

This seems to have broken unit-tests with default features for me:

cargo test yields:

   Compiling futures-preview v0.3.0-alpha.19 (C:\Users\matth\Code\rust\futures-rs\futures)
error[E0432]: unresolved import `futures::executor::ThreadPool`
 --> futures\tests\eventual.rs:2:5
  |
2 | use futures::executor::ThreadPool;
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ no `ThreadPool` in `executor`

error[E0433]: failed to resolve: could not find `ThreadPool` in `executor`
  --> futures\tests\mutex.rs:39:39
   |
39 |     let mut pool = futures::executor::ThreadPool::builder()
   |                                       ^^^^^^^^^^ could not find `ThreadPool` in `executor`

Is there something missing, that it gets enabled for test builds again?

[taiki-e]

We only test with --all-features, so it is recommended to pass --all-features flag when running tests locally.
https://github.com/rust-lang-nursery/futures-rs/blob/99d5ebed1288fe61e21151c86b2223e8872070c0/.travis.yml#L200-L201