XS daemons should degrade gracefully under load or hardware limitations

[mchua]

(Brought up at the XS/tech support meeting at 1cc on 12 November 2008, listing here for documentation purposes. It is my understanding that this work is ongoing.)

Work on the XS software to make the daemons degrade gracefully when XS-capacity-per-XO drops for any reason, be that a larger number of XOs connecting to an XS, or a physical XS machine with less-than-optimal hardware specs.

[mstone-xmlrpc]

Pushing out to 9.1.0, per edmcnierney's request.