B-77840: read XSLT file during init() instead of ever request in doFilter()

An improvement suggested by @usnavi (thanks!). These XSLT files are small, 1.5k - 2k. It should be ok to keep in memory.
That's a great point! Although, I am seriously thinking I should back out the java.nio call I was using. I want us to have the option to go back down to JDK6. Since JDK7 was just deployed to Prod last Friday, it has not have lots of production traffic runtime yet.