Track modified size in TxMemPoolEntry to correctly compute priority

`CTxMemPoolEntry::GetPriority` was incorrectly using the full size of a transaction to caclulate a priority delta to figure out current priority as opposed to priority when the transaction entered the mempool.  I added `nModSize` to `CTxMemPoolEntry` and fixed `GetPriority`.
Can you refactor so this isn't repeated in txmempool.cpp and core.cpp? If not, then a comment in both places is in order, so if we decide it should be 42U + ... we update in both places.