fix(chain): introduce keychain-variant-ranking to `KeychainTxOutIndex`

fix(chain): introduce keychain-variant-ranking to `KeychainTxOutIndex`

This fixes the bug with changesets not being monotone. Previously, the
result of applying changesets individually v.s. applying the aggregate
of changesets may result in different `KeychainTxOutIndex` states.

The nature of the changeset allows different keychain types to share the
same descriptor. However, the previous design did not take this into
account. To do this properly, we should keep track of all keychains
currently associated with a given descriptor. However, the API only
allows returning one keychain per spk/txout/outpoint (which is a good
API).

Therefore, we rank keychain variants by `Ord`. Earlier keychain variants
have a higher rank, and the first keychain will be returned.