Make CTxIndex::vSpent private.

Make the CTxIndex::vSpent member private, and access it only via
interface functions from the outside.
Fix vSpent private merge.

Fix remaining usings of CTxIndex::vSpent in Huntercoin-specific code.
This requires adding an accessor to vSpent with disk position, to check
if the spending tx is a game tx.  This can be removed later on, though,
to allow for the actual optimisations.