Rather than individually calling addUnchecked for each transaction added in a
changeset (after removing all the to-be-removed transactions), instead we can
take advantage of boost::multi_index's splicing features to extract and insert
entries directly from the staging multi_index into mapTx.
This has the immediate advantage of saving allocation overhead for mempool
entries which have already been allocated once. This also means that the memory
locations of mempool entries will not change when transactions go from staging
to the main mempool.
Additionally, eliminate addUnchecked and require all new transactions to enter
the mempool via a CTxMemPoolChangeSet.
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.
