Since we have a simple way to query the database for UTXOs we can
simplify some of the coin selection logic. That gets rid of the
in-memory list of UTXOs.
Not the nicest code, but it allows us to store the bip32_max_index so
that we don't forget our addresses upon restart. We could have done
the same by retrieving the max index from our index, but then we'd
forget addresses that don't have an associated output. Conversion
to/from string is so that we can store arbitrary one off values in the
DB in the future, independent of type.
