Check server status in batch

And cache the results for 5 seconds.  This should remove a huge
amount of GETs for individual server status, instead replacing
them with a single request that fetches the status of all servers.
All individual build or delete threads will use the cached result
from the most recent server list, and if it is out of date, they
will trigger an update.

The main benefit of using the individual requests was that they
also provided progress information.  Since that just goes into
the logs and no one cares, we can certainly do without it.

Also includes a minor constant replacement.

Change-Id: I995c3f39e5c3cddc6f1b2ce6b91bcd178ef2fbb0