Tidy up memory stress test

[bbockelm]

In #1942 we noticed that the longer the Linux stress test runs, the more memory it uses -- indicating a leak or memory hoarding.

Indeed, running with pprof enabled, there were two places with lots of unexpected churn in the heap:

1. (Minor) We recompile the same User-Agent regexp for each request. This can be a global.
2. The various origin/cache monitoring caches were filling with temporary data. These ttlcaches would eventually empty out -- but more on the order of minutes, making the test unpredictable.