(dev/core#217) PrevNext - Probe for best available implementation (memory-backed...

(dev/core#217) PrevNext - Probe for best available implementation (memory-backed or SQL-backed)

Before
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The `prevnext` service is always an instance of `CRM_Core_PrevNextCache_Sql`.

After
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If you define `CIVICRM_DB_CACHE_CLASS`, and if there's a corresponding
service `prevnext.driver.{mydriver}`, then it will use that service.

Otherwise, it will use `prevnext.driver.sql` (`CRM_Core_PrevNextCache_Sql`).