Optimized OgnlContext constructors, reduced collection sizes

[danielfernandez]

I've optimized the OgnlContext constructors so that:

• The _values map object is not instantiated if not really needed. This map (which was a large one, 23 entries) was being always created, even if the OgnlContext constructor being used immediately replaced it by one of its arguments. I'm executing some thousands of OGNL expressions per second and I'm always using new OgnlContext(myOwnMap), so this meant a lot of spurious, large HashMap objects being created without any need and a waste in memory usage.
• The _typeStack and _accessorStack objects, of type List, were being initialized to their default size which is 10, but very few OGNL expressions actually use 10 stack positions. Normally they don't go over 2 or 3. So by reducing the initial size of those collections to 3, a noticeable save in memory could be obtained under heavy loads.

[lukaszlenart]

Looks good, let's roll!