Expose a field to modify the poll interval in the proxy library

As @WofWca pointed out in https://github.com/guardianproject/orbot/issues/1105, orbot proxies poll very frequently, more frequently than browser proxies, because they use the Snowflake Go library which was historically only used by standalone clients with more resources. We should add a field to the SnowflakeProxy struct to allow callers of the library, like iptproxy, to modify the poll interval.