- simplify progress output (no \r, no terminal size related tweaks)
- emit progress output via the logging system (so it does not use stderr
  of borg serve)
- progress code always logs a json string, the json has all needed
  to either do json log output or plain text log output.
- use formatters to generate plain or json output from that.
- clean up setup_logging
- use a StderrHandler that always uses the **current** sys.stderr
- tweak TestPassphrase to not accidentally trigger just because of seeing 12 in output

for normal borg command invocation:
- logging is set up in Archiver.run
- the atexit handler calls logging.shutdown when process terminates

for tests:
- Archiver.run called by exec_cmd
- no atexit handler executed as process lives on
- borg.logger.teardown (calls shutdown and configured=False) now
  called in exec_cmd

- tears down logging (so no new log output is generated afterwards)
- sends all queued log output
- then returns

also: make stdin_fd / stdout_fd instance variables