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

...omized
I haven't seen a significant performance improvement when buffer size is increased (not yet anyway, not tested it directly), so given 3.5.1 is a bug-fix release IMO it's either this low-impact/risk approach or leave this out.  Any reason not to do this for 3.5.x and improve implementation as you say for 3.6 (which will be required anyway, in order for this to be configured in <configuration> element).