perf: Maintain shared XML parser instance per thread
perf: Maintain shared time format instance per thread

  + Expose the time format as base DateFormat rather than the overly specific
SimpleDateFormat, which use might be replaced with more optimized
implementation.
Clean up in SymbolicPage.getChildren()
Cache page data once read. Avoid unnecessary I/O.
Update transparency on breadcrumbs bar
Revert "Implement cache on filesystem level. About 20% improvement."
Performance tests do not prove a good enough increase.

This reverts commit d8753d7f233726687bd507bd7d36466411a5508a.
Remove fitnesse.util.Cache. No use for this class.
Merge pull request #473 from stanio/master

Small performance optimizations