We not only need to remove transactions stored in the block, but also
invalidate some potential double spends caused by these transactions. Usually
new block contains a substantial number of transactions from the pool, so it's
easier to make one pass over it only keeping valid items rather than remove
them one by one and make an additional pass to recheck inputs/witnesses.
It doesn't harm as we have transactions naturally ordered by fee anyway and it
makes managing them a little easier. This also makes slices store item itself
instead of pointers to it which reduces the pressure on the memory subsystem.
They shouldn't depend on the chain state and for the same transaction they
should always produce the same result. Thus, it makes no sense recalculating
them over and over again.
Eliminate races between tx checks and adding them to the mempool, ensure the
chain doesn't change while we're working with the new tx. Ensure only one
block addition attempt could be in progress.
