Fix memory leak

Creating a new session each time S3Storage is instantiated creates a memory leak. It seems that S3Storage can get created a bunch of times (I'm seeing it get created again and again as my app runs) and a boto3's Session takes loads of memory (see boto/boto3#1670) so my app eventually runs out of memory. This should fix the issue while still avoiding using the same session across different threads.

django_s3_storage/storage.py

@@ -93,9 +93,16 @@ def open(self, mode="rb"):
             self.file = self._storage.open(self.name, mode).file
         return super().open(mode)
 
+
+class _LocalSession(local):
+    def __init__(self):
+        self.session = boto3.session.Session()
+
+
+local_session = _LocalSession()
 
-class _Local(local):
 
+class _Local(local):
     """
     Thread-local connection manager.
 
@@ -117,8 +124,7 @@ def __init__(self, storage):
             connection_kwargs["aws_session_token"] = storage.settings.AWS_SESSION_TOKEN
         if storage.settings.AWS_S3_ENDPOINT_URL:
             connection_kwargs["endpoint_url"] = storage.settings.AWS_S3_ENDPOINT_URL
-        self.session = boto3.session.Session()
-        self.s3_connection = self.session.client(
+        self.s3_connection = local_session.client(
             "s3",
             config=Config(
                 s3={"addressing_style": storage.settings.AWS_S3_ADDRESSING_STYLE},