Make locker indexing better targeted

[rickyrombo]

Somewhat reverts #548 - instead of listening to every locker that a mint has, listen to two canonical mints:

• The first is the one derived from the DBC, same as before
• The second is the one derived from the mint, which will be used as the base for bespoke coins

This is somewhat important as in query land we seem to assume that there's only one locker for a given mint, and that's the "artist locker"

[raymondjacobson]

nice nice