Add variant of InsertUtxo for whole transaction.

Add a new variant of CUtxoDB::InsertUtxo that inserts all outputs of a
given (new) transaction.
Partially update UTXO DB in ConnectInputs.

Add a new transaction's outputs to the UTXO DB in ConnectInputs and
check that its inputs are indeed in the DB.  We do not yet remove spent
outputs, though.