Field data "transaction" design upgrade

[seanpearsonuk]

As discussed:

• Cleaner interface to the transaction-based field-data -- similar to the clean-up to the direct interface.
• Consider the returned data object to be a custom-filtered snapshot of the full data. Therefore it makes sense for the returned transaction data to have the same interface as the full data.
• The transaction could be renamed to something clear and simple like FieldQuery.
• It might make some sense to implement FieldQuery in terms of the existing transaction code.
• We could consider having a formal field data abstract query interface in the code.