Re-enable RBF

[iljakuklic]

RBF (replace-by-fee) was disabled when mempool was ported over to use TransactionVerifier.

The major missing piece is the ability to verify transactions where some UTXOs have already been spent. Transaction verifier currently does not provide that functionality out of the box. A brute force approach would be to temporarily disconnect all conflicting transactions, verify the incoming transactions, and either connect it if it satisfies all the rules, or reject it and re-connect the conflicts.

The origianl code is still present but is currently guarded by the ENABLE_RBF constant which is false. This makes it effectively dead code. There is also a number of disabled tests.