Fix peak-thread race on session-load

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.

libs/ardour/session.cc

@@ -424,6 +424,8 @@ Session::Session (AudioEngine &eng,
 			destroy ();
 			throw SessionException (_("Failed to load state"));
 		}
+
+		ensure_subdirs (); // archived or zipped sessions may lack peaks/ analysis/ etc
 	}
 
 	/* apply the loaded state_tree */
@@ -509,8 +511,6 @@ Session::Session (AudioEngine &eng,
 	_engine.set_session (this);
 	_engine.reset_timebase ();
 
-	ensure_subdirs (); // archived or zipped sessions may lack peaks/ analysis/ etc
-
 	if (!mix_template.empty ()) {
 		/* ::create() unsets _is_new after creating the session.
 		 * But for templated sessions, the sample-rate is initially unset

libs/ardour/source.cc

@@ -368,10 +368,6 @@ Source::get_transients_path () const
 	vector<string> parts;
 	string s;
 
-	/* old sessions may not have the analysis directory */
-
-	_session.ensure_subdirs ();
-
 	s = _session.analysis_dir ();
 	parts.push_back (s);