Combo SearchParameter improvements

[jamesagnew]

This PR cleans up the way that combo SPs are indexed and searched for:

• The non-unique combo param index table has a hash column that was previously not used. This is now indexed and searched against, and the old column will be dropped in a future release.
• The unique combo param index table did not have a hash column, so one has been added. In a future release it will be indexed and searched against, for now it is only being populated as the first step in a multi-release cleanup.
• Indexing for the two kinds of combo params has been harmonized, previously these were indexed in completely separate parts of the indexing pipeline. This does not affect functionality, but should make maintenance much easier.
• A number of tests have been added for non-unique combo params, as this feature was not well unit tested

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.42%. Comparing base (497b9f2) to head (e33d4a7).
Report is 53 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5885      +/-   ##
============================================
+ Coverage     83.39%   83.42%   +0.02%     
- Complexity    26927    27016      +89     
============================================
  Files          1681     1688       +7     
  Lines        103965   104297     +332     
  Branches      13189    13228      +39     
============================================
+ Hits          86702    87009     +307     
- Misses        11613    11635      +22     
- Partials       5650     5653       +3     

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