Track double-spends and RBF signaling

[OttoAllmendinger]

Transactions in the mempool can be replaced by other transactions. The official mechanism for this is Replace-By-Fee (RBF): https://github.com/bitcoin/bips/blob/master/bip-0125.mediawiki

However, miners have an incentive to use the highest-paying transactions even if they do not signal RBF.

We should track conflicting transactions in bademeister as well as the RBF signal (nSequence number).

Proposal

• add columns
  • nSequence INT
  • conflicts_with BLOB
• when a transaction tx_b conflicts with an existing transaction tx_a, add tx_b and set conficts_with to short(tx_a).
• any subsequent transactions that conflict with tx_b or tx_a will also set conflicts_with=short(tx_a).
• The transactions tx_a and all transactions that have conflicts_with=short(tx_a) are now a "conflict group"

When reconstructing the mempool:

• when processing a block confirmation, for all confirmed transactions: evict all transactions and their descendents that are in the same conflict group

[0xB10C]

As per BIP125 you can RBF a transaction that does not explicitly signal RBF via it's sequence number, if a parent transaction is replaceable via BIP125 RBF.

Exact wording:

• Explicit signaling: A transaction is considered to have opted in to allowing replacement of itself if any of its inputs have an nSequence number less than (0xffffffff - 1).
• Inherited signaling: Transactions that don't explicitly signal replaceability are replaceable under this policy for as long as any one of their ancestors signals replaceability and remains unconfirmed.

We should make sure to keep track of this as well.