allow config of cherrypy # threads via local_settings

Title says it all.  This addresses #533.

Code-wise, this a low-impact change (not only little code, but follows the logic of PRODUCTION_PORT (minus the command-line options, even!)).

The code is low-impact on code churn, but high impact on the performance side ( so says @gimick ).  I suggest we either merge this into 0.10.0 directly, or do this as a hotfix shortly after.  The RPi is our "star" platform.