fs: add support for non-threadpool executors (#1495)

Provides a thread pool dedicated to running blocking operations (#588)
and update `tokio-fs` to use this pool.

In an effort to make incremental progress, this is an initial step
towards a final solution. First, it provides a very basic pool
implementation with the intend that the pool will be
replaced before the final release. Second, it updates `tokio-fs` to
always use this blocking pool instead of conditionally using
`threadpool::blocking`. Issue #588 contains additional discussion around
potential improvements to the "blocking for all" strategy.

The implementation provided here builds on work started in #954 and
continued in #1045. The general idea is th same as #1045, but the PR
improves on some of the details:

* The number of explicit operations tracked by `File` is reduced only to
  the ones that could interact. All other ops are spawned on the
  blocking pool without being tracked by the `File` instance.

* The `seek` implementation is not backed by a trait and `poll_seek`
  function. This avoids the question of how to model non-blocking seeks
  on top of a blocking file. In this patch, `seek` is represented as an
  `async fn`. If the associated future is dropped before the caller
  observes the return value, we make no effort to define the state in
  which the file ends up.

tokio-executor/src/lib.rs

@@ -67,6 +67,9 @@ mod global;
 pub mod park;
 mod typed;
 
+#[cfg(feature = "blocking")]
+pub mod blocking;
+
 #[cfg(feature = "current-thread")]
 pub mod current_thread;