gddo-server: add server.configPoller

A config poller is added to the server, which polls for config data
based on the setting of GDDO_CONFIG_LOCATION.

Change-Id: Ib4330c3614a17e32930a09071043e5f2b50e6bf3
Reviewed-on: https://go-review.googlesource.com/c/gddo/+/285932
Trust: Julie Qiu <julie@golang.org>
Reviewed-by: Jonathan Amsterdam <jba@google.com>