Implement sampled timer

[andreynkravtsov]

Currently the timer implementation does not provide out-of-the-box sampling of the values and floods the aggregator if used in high QPS. There is an implementation of the sampled timer done at https://github.com/m3db/m3/blob/master/src/x/instrument/methods.go#L45

Perhaps we could pull that implementation into tally.

In addition, I've noticed that the implementation at the above URL lacks support for Stop() (only uses Record) due to null value of the recorder in the nullStopWatchStart, so a small tweak would need to be done if the solution is taken as-is.

nullStopWatchStart = tally.NewStopwatch(time.Time{}, nullRecorder{})
type nullRecorder struct{}
func (n nullRecorder) RecordStopwatch(stopwatchStart time.Time) {}

Else current Stop() of the Stopwatch fails due to null pointer use