Detect when an older state version is present, and delete it

[teor2345]

Is your feature request related to a problem? Please describe.

The plan for future database format changes is to blow away the database and start over. Because the database is a cache of chain state, we don't lose any information by doing so, and it's much simpler and more reliable than trying to perform a migration.

Originally posted by @hdevalence in #1172 (comment)

Describe the solution you'd like

Delete any previous versions on database load:

• {cache_dir}/zebra/v{number}/{network}
  • If Zebra is running version n, we must not delete versions greater than n
  • If possible, restrict the deletion to one filesystem, and don't follow symlinks

Optional:

• Add a debug config that turns this behaviour off, so developers can keep old states around

Note: number starts at 0

Scheduling

This ticket is only required for the first zebra-state database format change after the Zebra stable release.

[teor2345]

We might want to do this ticket for NU5, but it will only be needed if we do a rollback or a new state version.

[teor2345]

We might need to do this to support ZecWallet and lightwalletd, let's ask Aditya.