Whitelist and/or discounting privacy transactions

[luke-jr]

Currently, Samourai privacy transactions (tx0) are using over the default datacarriersize limit of 42 bytes. This isn't ideal, and while they should really fix their end to not require datacarrier at all, it makes sense to whitelist it until then.

This needs some way to differentiate their tx0 from other datacarrier txs.

Maybe it would make sense to have an option to then prioritise them further (but that might not be fair unless it can detect other privacy txs too)