Another attempt at abstracting Instant::now (#381)

Currently, the timer uses a `Now` trait to abstract the source of time.
This allows time to be mocked out. However, the current implementation
has a number of limitations as represented by #288 and #296.

The main issues are that `Now` requires `&mut self` which prevents a
value from being easily used in a concurrent environment. Also, when
wanting to write code that is abstract over the source of time, generics
get out of hand.

This patch provides an alternate solution. A new type, `Clock` is
provided which defaults to `Instant::now` as the source of time, but
allows configuring the actual source using a new iteration of the `Now`
trait. This time, `Now` is `Send + Sync + 'static`. Internally, `Clock`
stores the now value in an `Arc<Now>` value, which introduces dynamism
and allows `Clock` values to be cloned and be `Sync`.

Also, the current clock can be set for the current execution context
using the `with_default` pattern.

Because using the `Instant::now` will be the most common case by far, it
is special cased in order to avoid the need to allocate an `Arc` and use
dynamic dispatch.

src/clock.rs

@@ -0,0 +1,15 @@
+//! A configurable source of time.
+//!
+//! This module provides the [`now`][n] function, which returns an `Instant`
+//! representing "now". The source of time used by this function is configurable
+//! (via the [`tokio-timer`] crate) and allows mocking out the source of time in
+//! tests or performing caching operations to reduce the number of syscalls.
+//!
+//! Note that, because the source of time is configurable, it is possible to
+//! observe non-monotonic behavior when calling [`now`] from different
+//! executors.
+//!
+//! [n]: fn.now.html
+//! [`tokio-timer`]: https://docs.rs/tokio-timer/0.2/tokio_timer/clock/index.html
+
+pub use tokio_timer::clock::now;