MULE-7664 Allow size of internal buffer used for copy streams to be cust...

...omized
IMHO it's better to introduce the streamCopier. If we decide later to actually create the StreamCopier, you still need to maintain this change to keep backwards against any client depending on this.

Introducing the StreamCopier in 3.5.1 is ok in my opinion because it's just a utility class, not a feature itself - specially when compared to what you're planning to use it for ;) -