a duplicate webmention is received. Post module now returns after
creating a new post and then sends webmentions from a callback.
Received webmentions are now handled by a queue, and webmentions are
also resent for links in a post when a comment is received. Reader
module also uses a queue to process received updates. Notifications
are now sent to users when a reply is received for any comments
that have been received via webmention.