Add blocking mode

[nicholaschiasson]

Depends on #48

Blocking mode? For real? Are you serious? I know, it sounds crazy, but I think it should be an option.

Add a directive attribute blocking which when passed will cause the peer init handler to use ngx_parse_url instead of the configured resolver.

The reasons I think this is important are a bit complicated and come down to DNS resolution stability differences between using differing resolvers during runtime... Using this blocking option would ensure the DNS resolution each interval uses the same resolver (I suppose the system one?) as the one used during initialization.