dashboard/watcher: use Gerrit's JSON meta URL to poll smarter

Reduces our HTTP requests to Gerrit by a factor of the number of
subrepos we have.

Change-Id: I3f8fabeb70fdb5c276c639924baebcf5510fda9b
Reviewed-on: https://go-review.googlesource.com/1568
Reviewed-by: Andrew Gerrand <adg@golang.org>
Reviewed-by: Chris Manghane <cmang@golang.org>