Eliminate off-server export code

- Completely eliminate export service from client interface.
- No longer advertise data sources in messages to off-server clients
- Nobody actually cares about the returned result from EE.exportAction,
  so make it go away.
- Remove dead code.
- Get rid of the message part of ExportProtoMessage.  Make the
  AdvertisedDataSource which everyone uses a first-class object.
- We don't need bootClient() any more since it was for off-server export
  only.

Has matching PRO pull request