RFR [9] 8148117: Move sun.misc.Cleaner to jdk.internal.ref

[Andrew Haley]

Here's a question for you: could you tolerate a closeable mapped
ByteBuffer which was somewhat slower to access?  It is hard to make a
secure closeable mapped ByteBuffer: the question is how small the
overhead of closeability can be made.

But if the answer is "We can't tolerate *any* additional overhead, no
matter how small" then there is no motivation to do any experiments.

Andrew.