Summary
-------

Overload MappedByteBuffer.force to specify a subregion of the mapped buffer to be written back to persistent storage. 

Problem
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The current force method only supports writeback of a buffer's full mapped region. Selective writeback will be more efficient for some current use cases where a buffer is backed by a disk file. It will be much more efficient for the new use case proposed in JEP 352 where the buffer is backed by NVRAM.

Solution
--------

Overload the method to allow a region of the buffer to be specified and provide an implementation 

Specification
-------------

Add the following new method with associated javadoc

    /**
     * Forces any changes made to a region of this buffer's content to
     * be written to the storage device containing the mapped
     * file. The region starts at the given {@code index} in this
     * buffer and is {@code length} bytes.
     *
     * <p> If the file mapped into this buffer resides on a local
     * storage device then when this method returns it is guaranteed
     * that all changes made to the selected region buffer since it
     * was created, or since this method was last invoked, will have
     * been written to that device. The force operation is free to
     * write bytes that lie outside the specified region, for example
     * to ensure that data blocks of some device-specific granularity
     * are transferred in their entirety.
     *
     * <p> If the file does not reside on a local device then no such
     * guarantee is made.
     *
     * <p> If this buffer was not mapped in read/write mode ({@link
     * java.nio.channels.FileChannel.MapMode#READ_WRITE}) then
     * invoking this method has no effect. </p>
     *
     * @param index
     *        The index of the first byte in the buffer region that is
     *        to be written back to storage; must be non-negative
     *        and less than limit()
     *
     * @param length
     *        The length of the region in bytes; must be non-negative
     *        and no larger than limit() - index
     *
     * @throws IndexOutOfBoundsException
     *         if the preconditions on the index and length do not
     *         hold.
     *
     * @return  This buffer
     *
     * @since 13
     */
    public final MappedByteBuffer force(int index, int length)
    . . .