Improve HTTP logging

Improve HTTP logging

* Log HTTP debug lines to the "http" channel instead of wfDebug()
* Add the ability to do debug logging to MultiHttpClient
* Add a static method Http::createMultiClient() which provides a
  MultiHttpClient configured similarly to the way individual requests
  are configured, respecting the wiki's $wgHTTPTimeout and writing debug
  logs.
* In EtcdConfig, pass the logger instance through to the MultiHttpClient
  backend.

Change-Id: Ic5bdcb0cae95d7b3715ab5261758be082751c3ff