Fix handling various date formats for DateHistogram source field in continuous Rollups

[qreshi]

Issue #, if available: #374

Description of changes:
Continuous rollups encountered parse exceptions for certain date formats, this fix converts
the date field to epoch millis before calculating start and end time so that the logic is date format agnostic.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov]

Codecov Report

Merging #385 (6405a06) into master (ff33f16) will increase coverage by 0.22%.
The diff coverage is 75.00%.

@@             Coverage Diff              @@
##             master     #385      +/-   ##
============================================
+ Coverage     76.29%   76.52%   +0.22%     
- Complexity     1454     1461       +7     
============================================
  Files           188      188              
  Lines          7332     7334       +2     
  Branches       1187     1188       +1     
============================================
+ Hits           5594     5612      +18     
+ Misses         1097     1081      -16     
  Partials        641      641              

Impacted Files	Coverage Δ	Complexity Δ
...ch/indexmanagement/rollup/RollupMetadataService.kt	75.24% <71.42%> (+6.09%)	34.00 <0.00> (+4.00)
...csearch/indexmanagement/rollup/util/RollupUtils.kt	80.49% <100.00%> (+2.58%)	0.00 <0.00> (ø)
...nt/indexstatemanagement/model/destination/Slack.kt	40.90% <0.00%> (-13.64%)	2.00% <0.00%> (-2.00%)
...adata/TransportUpdateManagedIndexMetaDataAction.kt	85.18% <0.00%> (-3.71%)	9.00% <0.00%> (-1.00%)
...agement/indexstatemanagement/ManagedIndexRunner.kt	55.04% <0.00%> (-0.62%)	41.00% <0.00%> (ø%)
...icsearch/indexmanagement/rollup/model/ISMRollup.kt	94.49% <0.00%> (+1.83%)	31.00% <0.00%> (+4.00%)
...nt/indexstatemanagement/model/destination/Chime.kt	54.54% <0.00%> (+13.63%)	4.00% <0.00%> (+2.00%)

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