Rework ConnectInputs to clarify structure.

Rewrite the CTransaction::ConnectInputs function a bit to clarify the
structure and help with future optimisations with respect to game
transactions.
Further rewrite ConnectInputs.

Rewrite ConnectInputs further.  This removes all quirks to build up
txindex.vSpent when fFound is false.  When we don't have a txindex yet,
vSpent actually isn't really used.  Make this explicit.