Experimental UTXO-based tx verification

This patch implements an experimental UTXO-based verification of transactions.  It does not do full "UTXO pruning" (i. e., disk space is not reduced), but it should improve performance of syncing.  With the patch, previous transactions need not be loaded in ConnectInputs from random disk positions.  This will be a base for future pruning of old blocks on disk.

Please do not yet merge it, but test it a bit - I'm running some of my clients with it now, too, and everything seems to work so far.  Especially reorganisations / DisconnectInputs needs more testing, though, to make sure everything is fine.