Idea: store fee xpub in database

[jholdstock]

Currently dcrvsp is loading a fee xpub from config every time it starts. I have two issues with this:

• If the operator changes the value set in config, the behaviour is undefined.
• It adds an extra step to recovering a backup. The operator has to put their backed up database in place, and also ensure the correct xpub is set in config.

Alternative: Accept an xpub once, first time a VSP is started, probably via CLI flag. Store this in the database. This fixes both issues above. Comparable to having to init dcrwallet the first time it is run, using --create