Fix aggregation with persistence store AGG_TIMESTAMP not applying for group by

[niveathika]

Purpose

$subject fix #1591

Security checks

• Followed secure coding standards in http://wso2.com/technical-reports/wso2-secure-engineering-guidelines? yes/no
• Ran FindSecurityBugs plugin and verified report? yes/no
• Confirmed that this PR doesn't commit any keys, passwords, tokens, usernames, or other secrets? yes/no

[codecov-io]

Codecov Report

Merging #1593 into master will increase coverage by 0.01%.
The diff coverage is 62.5%.

@@             Coverage Diff              @@
##             master    #1593      +/-   ##
============================================
+ Coverage      67.7%   67.72%   +0.01%     
  Complexity       45       45              
============================================
  Files           651      651              
  Lines         27489    27492       +3     
  Branches       4335     4336       +1     
============================================
+ Hits          18612    18619       +7     
+ Misses         7064     7059       -5     
- Partials       1813     1814       +1

Impacted Files	Coverage Δ	Complexity Δ
...io/siddhi/core/aggregation/AggregationRuntime.java	53.11% <62.5%> (+0.46%)	0 <0> (ø)	⬇️
...ore/util/persistence/InMemoryPersistenceStore.java	53.84% <0%> (-5.13%)	0% <0%> (ø)
...ut/stream/state/AbsentStreamPreStateProcessor.java	82.11% <0%> (-1.33%)	0% <0%> (ø)
.../io/siddhi/core/util/snapshot/SnapshotService.java	70.6% <0%> (-0.21%)	0% <0%> (ø)
...a/io/siddhi/core/query/selector/QuerySelector.java	82% <0%> (+0.69%)	0% <0%> (ø)	⬇️
...rocessor/stream/window/SessionWindowProcessor.java	66.41% <0%> (+1.49%)	0% <0%> (ø)	⬇️
...c/main/java/io/siddhi/core/util/ThreadBarrier.java	100% <0%> (+20%)	0% <0%> (ø)	⬇️

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