refs #6301
- change knex getter def to be configurable, else it is not testable
- remove exportPath and lang from config - neither are used
- add client_trusted_domains to tables which shouldn't be exported as there are no clients in the export
- change export signature to be an object with `doExport` function consistent with import & easier to test
- cleanup export code so it is clearer, easier to read & to test:
- use mapSeries instead of sequence
- use Promise.props instead of Promise.join
- split functionality into smaller functions
- add test coverage
