Minor tweaks to external handler defaults.

* Use the X-Forwarded-For header for the remote address.
* Reuse sockets
Wrap external handler with instrumentation.

Use the InstrumentedHandler wrapper to get more metrics for the external
handler.
Enable GZip of responses.

If a client asks for it, attempt to compress responses for common text
only filetypes. No User-Agents are excluded as all of them should
support gzip correctly.