Add overall timeout for requests. (#67)

This deprecates timeout_read() and timeout_write() in favor of
timeout(). The new timeout method on Request takes a Duration instead
of a number of milliseconds, and is measured against overall request
time, not per-read time.

Once a request is started, the timeout is turned into a deadline
specific to that call. The deadline is used in conjunction with the
new DeadlineStream class, which sets a timeout on each read according
to the remaining time for the request. Once the request is done,
the DeadlineStream is unwrapped via .into::<Stream>() to become
an undecorated Stream again for return to the pool. Timeouts on the
stream are unset at this point.

Still to be done:

Add a setting on Agent for default timeout.
Change header-writing code to apply overall deadline rather than
per-write timeout.
Fixes #28.

src/test/mod.rs

@@ -14,6 +14,7 @@ mod query_string;
 mod range;
 mod redirect;
 mod simple;
+mod timeout;
 
 type RequestHandler = dyn Fn(&Unit) -> Result<Stream, Error> + Send + 'static;