Use a local extension trait instead of deprecated BoxFuture

[debris]

rust-lang/futures-rs#228

get rid of warnings in p2p and sync crates

warning: use of deprecated item: removed without replacement, recommended to use a local extension trait or function if needed, more details in https://github.com/alexcrichton/futures-rs/issues/228
 --> p2p/src/io/deadline.rs:3:31
  |
3 | use futures::{Future, Select, BoxFuture, Poll, Async};
  |                               ^^^^^^^^^
  |
  = note: #[warn(deprecated)] on by default

warning: use of deprecated item: removed without replacement, recommended to use a local extension trait or function if needed, more details in https://github.com/alexcrichton/futures-rs/issues/228
 --> p2p/src/io/deadline.rs:6:23
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6 | type DeadlineBox<F> = BoxFuture<DeadlineStatus<<F as Future>::Item>, <F as Future>::Error>;
  |                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

warning: use of deprecated item: removed without replacement, recommended to use a local extension trait or function if needed, more details in https://github.com/alexcrichton/futures-rs/issues/228
 --> p2p/src/p2p.rs:5:41
  |
5 | use futures::{Future, finished, failed, BoxFuture};
  |