New Feature: "Purge Holds"
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Similar to purging circulations one may wish to purge old (filled or canceled) hold information. This feature adds a database function and settings for doing so.

Purged holds are moved to the action.aged_hold_request table with patron identifying information scrubbed, much like circulations are moved to action.aged_circulation.

The settings allow for a default retention age as well as filled, canceled, and canceled by cancel cause ages. The most specific one wins unless a patron is retaining their hold history. In the latter case the patron's holds are retained either way.

Note that the function still needs to be called, which could be set up as a cron job or done more manually, say after statistics collection. A new script, purge_holds.srfsh, is added that can be used to purge holds from cron.