Remove global state about configuration and logs

Many things have been changed to make this possible, probably leading to
many hidden bugs waiting to be found.

Related to #122.

tests/custom/storage.py

@@ -24,5 +24,10 @@ Copy of filesystem storage backend for testing
 from radicale import storage
 
 
+# TODO: make something more in this collection (and test it)
 class Collection(storage.Collection):
     """Collection stored in a folder."""
+    def __init__(self, path, principal=False):
+        super().__init__(path, principal)
+        self._filesystem_path = storage.path_to_filesystem(
+            self.configuration.get("storage", "test_folder"), self.path)