fix(ckbtc/ckdoge): ensure that the fee rate of replacement transactions increases

[gregorydemay]

Follow-up on #8214 to ensure the fee rate of a replacement transaction always increases. Although not a requirement of BIP-125, this constraint is actually enforced by Dogecoin and Bitcoin miners and is part of the implementation referenced in the BIP

Replaces transactions already in the mempool if a new transaction seen with a higher fee, specifically both a higher fee per KB and a higher absolute fee.

This PR ensures this by

1. Recording the effective fee rate of a sent transaction instead of the initially estimated fee rate. The effective fee rate may be higher than the estimated one due the encoded signatures of the sent transaction requiring potentially less bytes than what was estimated (DER encoding of signatures is not constant size). This ensures that adding the relay fee for the replacement transaction does indeed increase the fee rate of the replacement transaction.
2. Rounding the estimated fee up to ensure that the effective fee rate is always at least the estimated one.

[mducroux]

Thanks for the fix @gregorydemay. LGTM!

[ninegua]

LGTM! Thanks