log-generator: add option to generate easily compressible data

log-generator: add option to generate easily compressible data

This is useful to check that compression actually works, and how
compression influences file size in the best-case-scenario for
compression. (The answer is that not as much as one would hope:
there's still a big overhead of the indexing and since every field
is compressed separately, even fields that compress very well
contribute to the file size. This overhead becomes negligible only
for very big fields.)