libstore: Raise default connect-timeout to 15 secs

This allows the weird network or DNS server fallback mechanism inside
glibc to work, and prevents a "Resolving timed out after 5000
milliseconds" error. Read on for details.

The DNS request stuff (dns-hosts) in glibc uses this fallback procedure
to minimize network RTT in the ideal case while dealing with
ill-behaving networks and DNS servers gracefully (see resolv.conf(5)):

- Use sendmmsg() to send UDP DNS requests for IPv4 and IPv6 in parallel
- If that times out (meaning that none or only one of the responses have
  been received), send the requests one by one, waiting for the response
  before sending the next request ("single-request")
- If that still times out, try to use a different socket (hence
  different address) for each request ("single-request-reopen")

The default timeout inside glibc is 5 seconds. Therefore, setting
connect-timeout, and therefore CURLOPT_CONNECTTIMEOUT to 5 seconds
prevents the single-request fallback, and setting it to even 10 seconds
prevents the single-request-reopen fallback as well.

The fallback decision is saved by glibc, but only thread-locally, and
libcurl starts a new thread for getaddrinfo() for each connection.
Therefore for every connection the fallback starts from sendmmsg() all
over again. And since these are considered to have timed out by libcurl,
even though getaddrinfo() might return a successful result, it is not
cached in libcurl.

While a user could tweak these with resolv.conf(5) options (e.g. using
networking.resolvconf.extraOptions in NixOS), and indeed that is
probably needed to avoid annoying delays, it still means that the
default connect-timeout of 5 is too low. Raise it to give fallback a
chance.

src/libstore/include/nix/store/filetransfer.hh

@@ -31,9 +31,17 @@ struct FileTransferSettings : Config
         )",
         {"binary-caches-parallel-connections"}};
 
+    /* Do not set this too low. On glibc, getaddrinfo() contains fallback code
+       paths that deal with ill-behaved DNS servers. Setting this too low
+       prevents some fallbacks from occurring.
+
+       See description of options timeout, single-request, single-request-reopen
+       in resolv.conf(5). Also see https://github.com/NixOS/nix/pull/13985 for
+       details on the interaction between getaddrinfo(3) behavior and libcurl
+       CURLOPT_CONNECTTIMEOUT. */
     Setting<unsigned long> connectTimeout{
         this,
-        5,
+        15,
         "connect-timeout",
         R"(
           The timeout (in seconds) for establishing connections in the