replacing lists with sets in offline file reader

to optimise the performance of OfflineReader, the keys must be stored in sets instead of lists.

added performance label

changed title from replacing lists by sets in offline file reader to replacing lists with sets in offline file reader

Or at least we should check which lists are used often in lookups, like if ... in ...

Sure! I remember our talk, I just needed to create an issue so that I don't forget about doing this work ;)

mentioned in merge request !15 (merged)

closed