Prior to this change, Spring's MethodParameter#getParameterAnnotations
called java.lang.Method#getParameterAnnotations on every invocation.
The latter ends up contending for a monitor inside (Sun) JDK code. This
is problematic when dealing with the high number of @RequestMapping
invocations that can occur in a Spring MVC @Controller.

This commit eliminates this contention by caching values returned by
java.lang.Method#getParameterAnnotations in a static ConcurrentMap.

Note that only Method parameter annotations are cached, while
Constructor parameter annotations are not. This is because the
issue of primary concern is, as mentioned above, @RequestMapping
methods. By nature, constructors are called much more infrequently, and
in most cases in a single-threaded fashion.

Issue: SPR-9298