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.
Make checks more strict.

Make the checks in (Dis)connectInputsGameTx stricter.  This enforces
more assumptions on game transactions, which should, however, be
satisfied in practice.