Xlate cache v0.5

Looks like I have this based on top of the "remove the flow dumper thread", which is as yet not merged. (It is on the mailing list for review). Please review the following four patches:

* bond: Export bond hashing functions for accounting.
* ofproto: New function ofproto_refresh_rule().
* ofproto-dpif: Allow caching of xlate_actions() effects.
* revalidator: Only revalidate high-throughput flows.

(I realised that below, the patches are out of order. d'oh.)

Basic look, 11 handlers, 5 revalidators
301e7cb: 78,000 flows, fairly stable (no handler CPU usage), 1 second dump duration.
132305c: 116,000 flows, fairly stable, 450-500ms dump duration. I had trouble stabilising higher than this.

I'll keep looking at this.