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.
Fixes a bug in the mini_miner_selection fuzz test found by fuzzing:
It was possible for the mini_miner_selection fuzz test to generated
transactions that created fewer new spendable outputs than the two
inputs they each spend. If the fuzz seed did so consistently, eventually
it would cause a `pop_front()` on an empty available_coins.
Fixed by:
- asserting that available_coins is not empty before generating tx
- allowing to build tx with a single coin if only one is available
