fix use-after-free of Task with futures2

[kpp]

Code stolen from #254

Any way there is error after #300:

$ cargo test --all --features="unstable-futures"  -j1
   Compiling tokio-threadpool v0.1.2 (file:///home/humbug/tokio/tokio-threadpool)
error[E0560]: struct `task::Task` has no field named `ptr`
  --> tokio-threadpool/src/task/mod.rs:92:16
   |
92 |         Task { ptr: Box::into_raw(inner) }
   |                ^^^ `task::Task` does not have this field
   |
   = note: available fields are: `state`, `next`, `future`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0560`.
error: Could not compile `tokio-threadpool`.

To learn more, run the command again with --verbose.

Also test by @seanmonstar fails:

$ cargo test --all -j1
...
...
test tests::task_drop_counts ... FAILED

failures:

---- tests::task_drop_counts stdout ----
	thread 'tests::task_drop_counts' panicked at 'assertion failed: `(left == right)`
  left: `0`,
 right: `1`', tokio-threadpool/src/lib.rs:95:9
note: Run with `RUST_BACKTRACE=1` for a backtrace.


failures:
    tests::task_drop_counts

[seanmonstar]

The tests are probably no longer relevant, since Task no longer manages it's ref counts manually, but just uses Arc. They could probably be removed...

[kpp]

They could probably be removed...

I love tests. The more the better. So I would be very grateful if you could fix them please.

[seanmonstar]

I like tests too. But I frankly don't understand the changes myself since I wrote the original patch.

[carllerche]

Thanks for this. However, the fix probably should be rethought as the threadpool internals changed. I don't even think the futures2 feature flag compiles.

Anyway, I'm going to close this PR in an effort to keep things organized.