TriggerMenuMT: speedup JSON generation

Frank Winklmeierrequested to merge
fwinkl/athena:menu_json into main

Store chain names in set to speedup lookup. Reduced the time to write the Dev_pp menu JSON from 1 minute to 5 seconds.

Also replace obsolete OrderedDict with dict as the latter is guaranteed to be ordered in Python 3.

Merge request reports