filtermail: run CPU-intensive handle_DATA in a thread pool executor

See
<https://docs.python.org/3/library/asyncio-eventloop.html#executing-code-in-thread-or-process-pools>
for the documentation.

This should avoid processing of large messages from hogging asyncio
thread and delaying async operations like accepting new connections.

CHANGELOG.md

@@ -2,6 +2,9 @@
 
 ## untagged
 
+- filtermail: run CPU-intensive handle_DATA in a thread pool executor
+  ([#676](https://github.com/chatmail/relay/pull/676))
+
 - don't use the complicated logging module in filtermail to exclude a potential source of errors. 
   ([#674](https://github.com/chatmail/relay/pull/674))