Assessing offered utxos

[AdamISZ]

This is more blue-sky thinking for now, but a client may want to assess the utxos it's going to be provided in a swap. Doing this without creating a snooping problem is non-trivial, probably impossible ( even cryptographic proof that utxos are not connected, let alone anything else, sounds pretty hard!), except: servers could be assessed on reputation; in some models clients might be, too.

What kind of utxos does the client want in its new history? Fresh ones obviously are desirable; perhaps mixed ones (from coinjoins - hence joinmarket) are better than ordinary-looking ones (which blockchain analysts may have flagged). Or perhaps the opposite.

Slightly comic thought is that a client may end up getting utxos that are actually strongly correlated with those it already has, defeating (partially) the point.