Fix issues identified during PR review by RyanjDick and brandonrising

- ModelMetadataStoreService is now injected into ModelRecordStoreService
  (these two services are really joined at the hip, and should someday be merged)
- ModelRecordStoreService is now injected into ModelManagerService
- Reduced timeout value for the various installer and download wait*() methods
- Introduced a Mock modelmanager for testing
- Removed bare print() statement with _logger in the install helper backend.
- Removed unused code from model loader init file
- Made `locker` a private variable in the `LoadedModel` object.
- Fixed up model merge frontend (will be deprecated anyway!)

invokeai/backend/model_manager/merge.py

@@ -39,10 +39,7 @@ class ModelMerger(object):
 
     def __init__(self, installer: ModelInstallServiceBase):
         """
-        Initialize a ModelMerger object.
-
-        :param store: Underlying storage manager for the running process.
-        :param config: InvokeAIAppConfig object (if not provided, default will be selected).
+        Initialize a ModelMerger object with the model installer.
         """
         self._installer = installer