Make PathPatternParser multi-threaded

With this change the original PathPatternParser is renamed
InternalPathPatternParser and a new PathPatternParser class is added.
This new PathPatternParser class is a very simple thread-safe wrapper for
the InternalPathPatternParser. It achieves this by creating a new
InternalPathPatternParser for each new parse request.  This follows the
model used for SpEL parsing.

Author: aclement