It's preferable to use type-safe transaction identifiers to avoid
confusing txid and wtxid. The next commit will add a reference to this
set; we use this opportunity to change it to Txid ahead of time instead
of adding new uses of uint256.
Since the kernel library no longer depends on the system file, move it
to the common library instead in accordance to the diagram in
doc/design/libraries.md.
Test each component of the RBF policy in isolation. Unlike the RBF
functional tests, these do not rely on things like RPC results, mempool
submission, etc.
