Bound the IP lookup's address list and name lookups (closes #110)

An IP lookup took any number of addresses and host names, gave each PTR
lookup until the request's 30-second limit, and looked up and named
every address a host name resolved to. It now answers 400 for more than
10 entries, or more than 10 addresses once the names are resolved,
before reading the database. Each PTR lookup gets 2 seconds; one that
runs out leaves the address's result without PTR names. Of a host
name's addresses only public ones are kept: global unicast and not
private. A name with none is reported as having no public address. The
tests give the server a fake resolver.

Model: opus-5-5

README.md

@@ -64,11 +64,16 @@ DEBUG=routewatch ./routewatch
   collector, stream and handler queue statistics
 - `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 (each address it resolves to
-  is looked up), or a comma-separated list of these. `results` has one entry per
-  address with a live route and `errors` one per address or name that failed.
-  The answer is 400 when nothing in `{ip}` is an IP address or a host name that
-  resolves, and 404 when none of the addresses has a live route.
+  PTR names. `{ip}` is an IP address, a host name, or a comma-separated list of
+  at most 10 of these. Of the addresses a host name resolves to, only public
+  ones are looked up: a name with none, such as one that resolves to a private
+  address, is reported as `host name has no public address`, without its
+  addresses. `results` has one entry per address with a live route and `errors`
+  one per address or name that failed. The answer is 400 when `{ip}` lists more
+  than 10, when it comes to more than 10 addresses once its names are resolved,
+  or when nothing in it is an IP address or a host name with a public address;
+  and 404 when none of the addresses has a live route. A PTR lookup gets 2
+  seconds; an address whose lookup takes longer is answered without PTR names.
 - `GET /api/v1/as/{asn}` - The AS and the prefixes it originates, IPv4 and IPv6
   apart, each with when its newest route was updated
 - `GET /api/v1/prefix/{prefix}/{length}` - Every live route for the prefix, one