Further update blkindex.dat by changing the vSpent format

This patch replaces the vSpent array in CTxIndex by a bit vector.  The only information actually used in the code is whether or not an output is spent, so this is enough.  This halves the size of blkindex.dat again.  I'm not aware of any memory or start-up improvements, though, but presumably it helps with general operation and blockchain syncing.  (I have no 'measurable results' to show, though, besides the storage reduction.)