(bf) Long chains created by direct Buffer::slice

A direct ByteBuffer created from another direct ByteBuffer, via slice say, will use the source buffer as an attachment for liveness rather than the source buffer's attachment.

This can result in long chains of buffers being kept alive that may induce ouf of memory exceptions.

For more discussion see:

  http://mail.openjdk.java.net/pipermail/core-libs-dev/2018-July/054515.html