Switch buildershandler over to using the json mirror.

The json mirror is apparently better able to scale to load
and is less dependent on the buildbot master being in
good state.

As part of this, we how have a way of querying the step
information for the last build on a builder, so we can
skip the request and code related to finding out the
build number of the latest run.

Change the masters data structure to store a url name
instead of the whole URL. The url name is the master name
to use for constructing URLs. This way buildershandler
can use the mirror, but builders.js can continue pointing
to the actual master for it's non-json queries.

Also, get rid of the force_update code path in buildershandler.
This seemed like a good idea at the time, but in practice it
just masks errors by making it seem like things are working.
The downside of deleting this is that persistent errors loading
the JSON won't be noticed until the memcache is flushed for
some reason. We should add monitoring of 500s to catch these
things earlier.

Review URL: https://codereview.chromium.org/337433003

git-svn-id: svn://svn.chromium.org/blink/trunk@176147 bbb929c8-8fbe-4397-9dbb-9b2b20218538