Don't pipe JSON outputs trough tracing

[WolverinDEV]

One example of json output is conserve debug index /backup/thing, which emits json. I see you added a -R so that you can turn it off, which is OK, but it just feels like it should be a different internal interface for "write a block of text, with no interruptions, to stdout".

However this PR is already very big so perhaps that should be left for later.

Originally posted by @sourcefrog in https://github.com/sourcefrog/conserve/pull/173#discussion_r945174822