(dev/core#217) PrevNext - More conservative transition (5.7=>5.8=>5.9)

(dev/core#217) PrevNext - More conservative transition (5.7=>5.8=>5.9)

This revision sets up more conservative transition process for adopting PrevNext drivers.

Before
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* 5.7.x - Hard-coded to SQL driver.
* 5.8.x - Allow setting to pick driver. If left as `default`, then choose best-available (based on configured services).

After
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* 5.7.x - Hard-coded to SQL driver.
* 5.8.x - Allow setting to pick driver. If left as `default`, then use SQL driver.
* 5.9.x - Allow setting to pick driver. If left as `default`, then choose best-available (based on configured services).

This essentially mitigates the risk that bugs in the new Redis driver cause regreessions for sites already running Redis.