**Warning: This commit contains a database upgrade.**
In order to add future support for retiring xpub keys, the database is
upgraded such that the keys are now stored in a dedicated bucket which
can hold multiple values rather than storing a single key as individual
values in the root bucket.
A new ID field is added to distinguish between keys. This ID is added to
every ticket record in the database in order to track which pubkey was
used for each ticket.
A new field named "Retired" has also been added to pubkeys. It is a unix
timestamp representing the moment the key was retired, or zero for the
currently active key.
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