java-idp-jetty-base: persistTempDirectory in idp.xml for Jetty 12

[Cantor, Scott]

Initial teting suggests Jetty 12.0.13 fixes this tmp cleanup bug and the regression they created in 12.0.12 trying to fix it, at least on Linux.

We'll be getting a Jetty 12 installer for Windows based on that version out next week, and then get back to working towards producing a full on IdP plugin that installs it all for people on either OS.

-- Scott