Don't expire unused relay-to-relay TLS conns so quickly

In git commit 67b38d50 we changed conn timeouts so relays would close idle conns (that is, conns without any circs on them) after 3 minutes rather than 15 minutes.

We made the change because of the "clients holding their dir-fetching conns open for a long time, filling up descriptor lists and memory and knocking over relays" fun: http://archives.seul.org/tor/relays/Apr-2010/msg00073.html

But it would appear that we made worse the problem that Torscan exploits. We don't need to be so aggressive about closing connections to/from other relays (besides, they weren't the problem before).

There's a downside here, which is that we end up using more file descriptors on relays. But if most links are used already, we don't use many more. And if most links aren't used already, the Torscan problems are worse.