prevHash == input.PrevHash, so make less DB accesses and more real work. Fix
some bugs along the way:
* spentCoins structure may already be present in the DB when persisting TX,
there is nothing wrong with that and we shouldn't overwrite it
* it's only used for NEO and only to check for claim validity. Thus, when
processing claim tx the corresponding spentCoins should always be present
in the DB
It's useless work being done before it's actually needed. These (updated with
new values) are going to be written with some kind of Put anyway, so writing
them here is just a waste of time.
