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

...omized
Not enough information currently, but with this change we can more easily do further testing, and will have the flexibility to tweak this for scenarios with large payloads.  On the contrary we are stuck with 4K buffer until at least 3.5.2.  The goal of this PR to start with was to expose this via an undocumented system property to give flexibility we don't currently have rather than as a new documented feature.