Cleanup contribution search by making non-exportable fields exportable

I've been looking into some performance issues & have been struggling with unnecessary hackiness in the code.

After some analysis I found that 4 fields in the Contribution schema have not been declared as exportable. In each case some hackery has been done to declare them after the fact. The fields are

payment_instrument_id
contribution_recur_id
financial_type_id
address_id

The first 3 of these have been hacked into the BAO::exportableFields function - which is the only place affected by the 'export' flag. Thus changing the schema to declare them as exportable will have no change other than requiring some code cleanup.

The last one, address_id is hacked into the contribution search. Adding export=True to this one will mean the address id can be searched on in search builder & exported. While I can't see a particular use for this I can't see a downside either.