Use a threadpool to fetch the files

See https://docs.aws.amazon.com/prescriptive-guidance/latest/patterns/run-parallel-reads-of-s3-objects-by-using-python-in-an-aws-lambda-function.html

src/transferrer/common.py

@@ -4,6 +4,7 @@
 
 SOURCE_BUCKET = "wellcomecollection-editorial-photography"
 
+
 def get_source_bucket(max_connections=10):
     return get_source_client(max_connections).Bucket(SOURCE_BUCKET)