Session dirs must exist when loading Sources of a project.
SourceFactory::init() starts the PeakFileBuilder background thread
early on when libardour is initialized.
This thread [re-]creates missing peak files when Sources are
created or loaded. However AudioSource::prepare_for_peakfile_writes
assumes the peak/ folder exists. This may not be true, which lead to
"AudioSource: cannot open _peakpath [..] (No such file or directory)"
errors.
Also drop creation from Source::get_transients_path() as this
may be called concurrently from Analyser background threads.
839400f438