metrics-core: Fix race condition in InstrumentedThreadFactoryTest.
Merge pull request #560 from udoprog/master

metrics-core: Fix race condition in InstrumentedThreadFactoryTest.
ExponentiallyDecayingReservoir: quantiles tests

* expect quantiles weighting
* check that reaction for mode change is within 10 minutes
Extract Snapshot interface and UniformSnapshot
Minor style changes
Add WeightedSnapshot and migrate ExponentiallyDecayingReservoir to it