wwwattach: support async blob retrievals

wwwattach: support async blob retrievals

We can reuse some of the GzipFilter infrastructure used by other
WWW components to handle slow blob retrieval, here.  The
difference from previous changes is we don't decide on the 200
status code until we've retrieved the blob and found the
attachment.

While we're at it, ensure we can compress text attachment
responses once again, since all text attachments are served
as text/plain.