Futures 0.2 integration issues

[carllerche]

This is a tracking issue for issues w/ the futures 0.2 integration:

• The sub dependencies aren't all correct. For example, when the futures2 feature is enabled for tokio-reactor, it does not enable it for tokio-executor.
• This test needs to be updated.
• Panic: 'unexpected task state; Complete' #248 seems to be caused by the futures 0.2 integration. @seanmonstar posted fix use-after-free of Task with futures2 #254 but it needs review.
• Re-enable tests disabled by Disable future 0.2 tests #259.
• StableFuture

cc/ @aturon

[kpp]

• Remove tokio-io/src/[read/read_exact...] in favour of https://github.com/rust-lang-nursery/futures-rs/tree/master/futures-util/src/io

[carllerche]

I also don't think that the futures2 crate is needed. Instead, Tokio can depend on the sub crates instead of the facade. This is probably going to be better practice anyway.

[carllerche]

I'm going to close this as it does not seem like Tokio will support futures 0.2. The feature flag has been removed and futures 0.3 is also being worked on.