Caching

[jamesbennet]

There are a few areas where we make a number of complex db queries, and produce some expensive-to-parse json (i.e unfinished jobs). I propose we memcache these, for performance. Been playing with it a lot lately, and it is nice. Probably want to cache both the DB calls and the json - essential if we want i.e 20+ buildds polling every 5 seconds, or some use case like that.