Add enums for keywords used in exit lists, Torperf measurement results, bridge pool assignments, and soon sanitized web logs
Todo derived from the discussion in comments 1 to 4:
Add keywords for all descriptors to Key enum.
Use a one letter prefix for keywords from non-Tor data sources.
First step: determine the best letter for each non-Tor source.
Original summary: We recently introduced the Key enum with keywords contained in relay descriptors, sanitized bridge descriptors, and sanitized bridge pool assignments. We did not include keywords in exit lists and Torperf measurement results, and in retrospect we should have excluded sanitized bridge pool assignments there.
The reason why each data source should have its own enum is that naming conventions might vary in terms of upper/lower case and word separators. For example, Tor descriptors use lower-case-keywords, exit lists contain CamelCase, and Torperf/OnionPerf use ALL_UPPER_CASE_WITH_UNDERSCORES. There could be conflicts for keywords like source vs. SOURCE.