T: fix deadlock when adding tracks

For livetrax, ::add_routes_inner calls ::reassign_track_numbers,
which in turn calls ::livetrax_queue_auto_connect.

This can result in a deadlock:

 * GUI thread, ::add_routes_inner holds the process-lock
   and ::livetrax_queue_auto_connect requests the `_auto_connect_queue_lock`

 * auto-connect thread first takes the `_auto_connect_queue_lock and then
   requests the process-lock

libs/ardour/session.cc

@@ -3681,6 +3681,8 @@ Session::add_routes_inner (RouteList& new_routes, bool input_auto_connect, bool
 					r->enable_direct_outs ();
 				}
 			}
+			/* do not take `_auto_connect_queue_lock' while holding the process lock */
+			lm.release ();
 			reassign_track_numbers ();
 		}
 	}