nntp: implement OVER/XOVER summary in search document
The document data of a search message already contains a good chunk
of the information needed to respond to OVER/XOVER commands quickly.
Expand on that and use the document data to implement OVER/XOVER
quickly.
This adds a dependency on Xapian being available for nntpd usage,
but is probably alright since nntpd is esoteric enough that anybody
willing to run nntpd will also want search functionality offered
by Xapian.
This also speeds up XHDR/HDR with the To: and Cc: headers and
:bytes/:lines article metadata used by some clients for header
displays and marking messages as read/unread.