don't overfill the prevnext cache when reserving for a survey.

[jmcclelland]

Overview

When reserving respondents for a survey, the civicrm_prevnext_cache table gets filled with all the contacts in the database rather then just the contacts returned by the search. This is barely noticeable on a small site or a site backed by a fast database. But on a big site or a slow database site it casuses a lot of churn and slowness.

Before

1. Create a Survey
2. Create a regular group with one person in it.
3. Check that the civicrm_prevnext_cache table is empty (or empty it)
4. Click to reserve respondents. Select your survey and group and click Search
5. Check the contents of the civicrm_prevnext_cache and you see the same number as total contacts in the civicrm_contact table instead of just one contact.

After

You should only see one record in the civicrm_nextprev_cache table.

Technical Details

I think this has been broken for a long time and it makes me wonder if the nextprev_cache is even used in this context.

[civibot]

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[eileenmcnaughton]

I feel like this functionality is not much used so I'm going with the low review bar of relying on your analysis & testing and just checking that your analysis is logical and the change looks sane - which it is & does. Merging