Try a global lock for all libraries to prevent more than one concurre…

…nt vopt operation.

This is a temp fix to try if that resolves identified problems

vunit/sim_if/modelsim.py

@@ -105,6 +105,7 @@ def __init__(self, prefix, output_path, persistent=False, gui=False):
         self._optimized_designs = {}
         self._optimized_libraries = {}
         self._vopt_lock = Lock()
+        self._library_lock = Lock()
 
     def _create_modelsim_ini(self):
         """
@@ -370,7 +371,7 @@ def _optimize(self, config, script_path):
                 optimize = True
 
                 if not self._optimized_libraries.get(config.library_name, None):
-                    self._optimized_libraries[config.library_name] = Lock()
+                    self._optimized_libraries[config.library_name] = self._library_lock
 
             simulation_target, vopt_event = self._optimized_designs[design_to_optimize].values()