Clear direct byte buffers

[garydgregory]

BufferedFileChannelInputStream now clears its direct memory byte buffer on close.

MemoryMappedFileInputStream now clears its direct memory byte buffer on close.

FileChannels.contentEquals(ReadableByteChannel, ReadableByteChannel, int) now clears its direct memory byte buffer on close.

BufferedFileChannelInputStream now clears its direct memory byte buffer on close.

MemoryMappedFileInputStream now clears its direct memory byte buffer on close.

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