Add more aggressive caching of previous txouts in CTxIn.

Cache previous txouts matching a CTxIn when they have been loaded from disk.  Also cache their height when it has been determined for CreateNewBlock.  This should speed CreateNewBlock (and thus getwork/getauxblock) up considerably for large numbers of inputs processed in a block.  Please review the code carefully to ensure I've not introduced any bugs, and test that all mining related things still work fine.  I've only done some casual testing that getwork and getauxblock still run fine, but can't really judge whether everything is fully correct.