Gh-3295: Fix issue with Accumulo iterators when smart merging Federated POC

[tb06904]

There was an issue where the iterators from Accumulo could be exhausted or closed before the merging of elements had completed. There's not a great way around this other than to buffer the data into a set before using for merging. This does of course increase memory usage but might be faster overall.
(Also the previous solution was still technically buffering to memory with a distict() call).

Related issue

• Resolve Issue with Accumulo iterators in federated POC #3295

[codecov]

Codecov Report

Attention: Patch coverage is 74.19355% with 8 lines in your changes missing coverage. Please review.

Project coverage is 67.77%. Comparing base (4ce1ec5) to head (d770cff).
Report is 22 commits behind head on develop.

Files with missing lines	Patch %	Lines
...imple/merge/operator/ElementAggregateOperator.java	63.15%	3 Missing and 4 partials ⚠️
...e/operation/handler/FederatedOperationHandler.java	80.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             develop    #3296   +/-   ##
==========================================
  Coverage      67.77%   67.77%           
  Complexity      2595     2595           
==========================================
  Files            949      949           
  Lines          30237    30242    +5     
  Branches        3339     3341    +2     
==========================================
+ Hits           20492    20497    +5     
+ Misses          8279     8278    -1     
- Partials        1466     1467    +1     

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[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud