Increase Snowflake proxy poll interval to 300 s

The Snowflake proxies are polling the broker more frequently than necessary for the number of clients we have. The broker reports about 500 proxies currently, which at the current poll interval of 20 s, is 25 polls/s.

At the anti-censorship meeting today we talked about increasing the interval to 300 s, which would still give us an expected time-to-proxy-assignment of less than 1 s.

17:41:02 <dcf1> A few weeks ago I tailed the log of the broker, and requests were coming in furiously.
17:41:23 <dcf1> Lately those particular log lines have been removed, so it's not as apparent, but according to https://snowflake-broker.bamsoftware.com/debug there's 500 proxies,
17:41:39 <dcf1> and with a poll interval of 20 s, that's 25 incoming proxy requests per second.
17:41:42 <arma2> i liked the idea of having the broker tell each snowflake when to come back
17:41:59 <dcf1> Something on the order of 1 or 2 per second is probably adequate.
17:42:07 <cohosh> arma2: serna has started on that ticket
17:42:14 <cohosh> i agree
17:42:23 <dcf1> arma2: yeah that's #25598, serna ran into some trouble with that.
17:42:42 <cohosh> we have metrics of how many idle proxies we have: https://metrics.torproject.org/collector/archive/snowflakes/
17:42:57 <cohosh> and it is orders of magnitude more than the the number of client matches
17:43:31 <dcf1> Anyway, I think an interval of around 300 seconds would be workable.
17:43:37 <cohosh> sounds good