This PR adds (preliminary?) support for ZKAPAuthorizer's new ZKAP Database Replication / Recovery feature, completing most -- but not all -- of the tasks outlined in this comment. The exceptions are:

• Increment Gridsync's rootcap version to "v1" (or "v2"?) to signal new/breaking functionality

This will be completed in the near future as part of addressing #449 (which may result in further breaking changes)

• Tests for all of the above

End-to-end/integration tests for ZKAP issuance/redemption/replication/recovery are notably lacking as a result of the complexities involved in spinning up a PaymentServer across (non-NixOS) platforms. Members of the PrivateStorage team are in the currently in the process of ameliorating this a) by developing separate tooling to automate the issuance/redemption of ZKAPs (among other things) and b) refactoring of some parts of the Gridsync codebase better facilitate headless end-to-end testing and automation.