Show route churn as a whole count per minute (closes #160)

The status page showed route churn per second as a one-minute moving
average, so a quiet feed gave a fraction such as 0.1. The average was
correct.

The metrics tracker now also counts prefix announcements and
withdrawals, from all peers, in each clock minute. /api/v1/stats
reports the count for the last full minute as route_churn_per_min in
stream, in place of route_churn_per_sec. The status page shows it as
"Route Churn/min".

The status and home pages round messages and updates per second to
whole numbers.

Model: opus-5-5

README.md

@@ -81,7 +81,9 @@ Every route answers `HEAD` as it answers `GET`, without the body.
 ### API v1
 
 - `GET /api/v1/stats` - The `/status.json` figures plus connection, garbage
-  collector, stream and handler queue statistics
+  collector, stream and handler queue statistics. `stream.route_churn_per_min`
+  is the number of prefix announcements and withdrawals, from all peers, in the
+  last full clock minute.
 - `GET /api/v1/ip/{ip}` - Looks up each address: its most specific live route
   (`netblock`), that route's origin AS with its WHOIS details, and the address's
   PTR names. `{ip}` is an IP address, a host name, or a comma-separated list of