This commit fixes a race/threading issue with the way the monitor
works. The osbuild monitor can be called from multiple threads,
e.g. in buildroot.py:run() monitor.log() is called but also
in host.py:_stdout_ready(). This can lead to out-of-order writes
when many messages need to be processed.
We did not notice this so far because we were lucky and also
log was just used for information. But now it is used to transmit
the jsonseq data which means out-of-order communication results
in broken json.
Closes: https://github.com/osbuild/image-builder-cli/issues/110
035781ea1c