minor tweaks related to jenkins slowness

Related to #1168, but does not resolve it.
* Installs timestamper plugin, which i installed by hand on jenkins, so some commit containing it should be approved to match the live state
* Tidies cleanup/recreate of build/ in build.xml
* Provides some prep/cleanup scripts for using a tmpfs for build/, so that if we ever find doing it that way to be a time savings, we can.  (Doesn't actually use those scripts in build.xml, because it turns out not to be a time savings right now.)