walletdb: change migration script

[tynes]

This PR moves the migration out of the node boot process and instead uses a script to do the migration. Users giving feedback seemed to like doing the migration through a script instead of through stopping, pulling a new version and restarting the node.

[pinheadmz]

I'm OK with this approach, from the developer call it sounds like any update where backing up the walletDB is prudent should not be done automatically, and we already discussed why rescanning automatically isn't a good idea either. I think this is good design practice for hsd going forward, although I'll also add here that in #396 the migration was executed automatically in updated software. In bcoin-org/bcoin#703 the user had to migrate the chainDB manually after updating bcoin, but before launching it again. If updated software is run without the migration, an assertion error stops the process: https://github.com/bcoin-org/bcoin/blob/master/lib/blockchain/chaindb.js#L63

So I think @tynes if you have the time what I think would be good here is this:

• In walletDB, check for the migration flag this.db.has(layout.M.encode(0)) in walletdb.open() and log an error. This will be output to debug log on every single launch of hsd until the issue is fixed. That error should either specify "see changelog" or directly reference the script migrate/wallet0.js.

• Add a description of the process (including how to back up the walletDB) in the CHANGELOG

• In the root README, right after INSTALL and a section UPDATING and if nothing else, just add "always read the changelog before updating"

Another option would be test for the migrate flag and instead of loging an error, just stop hsd with process.exit() and throw a descriptive error message -- thought on this @turbomaze ?

Will test the actual migration script shortly

[pinheadmz]

Hoping we can close this and rally around merging #415

[tynes]

Closing in favor of #415