Log IP addresses from HTTP requests

Querying /master/state.json is an expensive operation when a cluster is large, and it's possible to DOS the master via frequent and repeated queries (which is a separate problem). Querying the endpoint results in a log entry being written, but the entry lacks useful information, such as an IP address, response code and response size. These details are useful for tracking down who/what is querying the endpoint. Consider adding these details to the log entry, or even writing a separate access log. Also consider writing log entries for all HTTP requests (/metrics/snapshot produces no log entries).

I0319 18:06:18.824846 10521 http.cpp:478] HTTP request for '/master/state.json'