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.

tokio-timer/src/timer/mod.rs

@@ -27,6 +27,9 @@
 //! [`Now`]: trait.Now.html
 //! [`Now::now`]: trait.Now.html#method.now
 
+// This allows the usage of the old `Now` trait.
+#![allow(deprecated)]
+
 mod entry;
 mod handle;
 mod level;