Database Exporter: Allow export of big tables

[Hackwar]

Steps to reproduce the issue

Have a database with big tables and run the exporter on the CLI with a more or less restrictive memory limit. At some point you get "Allowed Memory Size" errors. We should have a look how to maybe write directly to a file instead of a buffer or to yield at some point to allow writing the buffer. To me the easiest solution seems to be to add an option to output to file pointer.