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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.