fix units on RescaleThreshold and initialize _nextScaleTime.

RescaleThreshold was being initialized to ns even though we were
comparing to ticks (100ns).

_nextScaleTime was not being initialized, so add that to Clear().

Conflicts:
	src/metrics/Stats/ExponentiallyDecayingSample.cs
improve seeding of ThreadLocalRandom

Conflicts:
	src/metrics/Support/ThreadLocalRandom.cs
Merge pull request #18 from slumos/rescale-time-fix

Rescale time fix