[FR] Opt-Out of Taproot

[shocknet-justin]

When upgrading to 15.x it is required to upgrade the back-end to something Taproot-aware, this can be a problem in production environments where multiple applications share a back-end.

The database upgrades in 15 being so compelling for production are at odds with the relatively bleeding-edge nature of Taproot, and personally it's benefits/risks are not fully understood.

A flag that allows upgrading of the database without all the Taproot implications would be ideal.