Fix time skew exceptions crashing disruptor thread

[jsuereth]

For now, we fix all known issues with time skew in Metric Views.

• In the event of a detected time skew, we just throw away all our data. We really don't know if it's safe to push anything we have, and we cannot safely do any aggregation / report correct timestamps.
• We add (the first?) tests for MutableViewData to ensure it does not throw when time rewinds, but instead drops recorded data points and resets itself.

Fixes #2068 (deemed to not have a workaround)

[codecov-io]

Codecov Report

Merging #2071 (abfb238) into master (6123b84) will decrease coverage by 0.01%. The diff coverage is 60.00%.

@@             Coverage Diff              @@
##             master    #2071      +/-   ##
============================================
- Coverage     83.08%   83.06%   -0.02%     
  Complexity     2264     2264              
============================================
  Files           322      322              
  Lines         10557    10565       +8     
  Branches       1048     1049       +1     
============================================
+ Hits           8771     8776       +5     
- Misses         1448     1451       +3     
  Partials        338      338              

Impacted Files	Coverage Δ	Complexity Δ
.../io/opencensus/implcore/stats/MutableViewData.java	92.07% <60.00%> (-1.52%)	4.00 <0.00> (ø)

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