debugindex: add a `p1-rev` column This will be useful in case of corrupted index.

debugindex: introduce a concept of "verbose-only" column We are about to add a bunch of new column and most of them are probably only relevant to --verbose. We add some more testing of the `--verbose` mode in a sidedata context.

debugindex: move to a flexible column Each column is now declared as a decorated function. This will make it much simpler to add more new column in the future.