Make the methodParameter field of HandlerMethod final

Previously the methodParameter array field was initialized lazily since
it requires reflection. However, in practice the field is always used
and there is not much benefit from the lazy initialization.

In Spring Framework 3.2, the methodParameter field was copied when a
new HandlerMethod instance (with the resolved bean) is created for
performance reasons. That introduced a synchronization issue since
the lazy initialization was not synchronized.

Issue: SPR-10365