While each git blob request is treated fairly w.r.t other git
blob requests, responses triggering thousands of git blob
requests can still noticeably increase latency for
less-expensive responses.
Move large mbox results and the nasty all.mbox endpoint to
a low priority queue which only fires once per-event loop
iteration. This reduces the response time of short HTTP
responses while many gigantic mboxes are being downloaded
simultaneously, but still maximizes use of available I/O
when there's no inexpensive HTTP responses happening.
This only affects PublicInbox::WWW users who use
public-inbox-httpd, not generic PSGI servers.