This is a small refactor to how we use the boost multi_index in CTxMemPool -- rather than have to use fixed index numbers in our code, I added some tags so that we can use logical names instead, so that e.g. nth_index<2> becomes index<entry_time>.
This was motivated by me rebasing some old work on to master, and I realized that the usage of fixed indices was error prone.
I didn't bother with a tag name for the zero-th index (txid), since nothing seems to explicitly use it, but I can add one if people think that'd be better.
