Allow using fee rate for claim

[i5hi]

Allow user to set fee_rate as an option for claim.

[salvatoto]

Would be useful. We had a race where coops were failing in some cases, and we actually tried to fall back to non-coop, which has a higher fee - the race was on our side.

Also, lowball vs non-lowball. If lowball fails for whatever reason we pass the higher absolute fee, but just passing the rate would be a lot safer.