Make the release exporter multi-threaded

[Nicklas Nordborg]

I think the performance of the release exporter can be improved by making it multi-threaded. Most of the work it is doing is for a single patient at a time so there should not be too many issues with synchronization.

[Nicklas Nordborg]

In 7022:

References #1453: Make the release exporter multi-threaded

Use 1/3 of the available cpus for the export. The ReleaseExporter will start multiple threads. It is important that all *Writer implementations are thread-safe.

[Nicklas Nordborg]

In 7034:

References #1453: Make the release exporter multi-threaded

Fixed sorting of files to make it easier to compare results when debugging.

[Nicklas Nordborg]

In 7035:

References #1453: Make the release exporter multi-threaded

Changed the JsonWriter implementation so that all JSON files are created by the main thread. This prevents an issue with the "local" ouput that tried to create the 'json' subdirectory multiple times resulting in an error.