Exporter cleanup & tests

[ErisDS]

This is a small cleanup / refactor which is a precursor to a bigger one for migrations. Doing this adds some ground work for testing other bits and pieces in migrations.

The main piece, other than the cleanup of the export code, is the change to the knex getter - setting configurable to be true rather than false, otherwise it is not possible to stub this method. IMO testability is more important here.

Two other changes are present:

• client trusted domains has been added to export exclude list as clients are already there
• two paths have been removed from the config, I am not sure that these have ever been used!

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