MappedByteBuffer*Stream:

- Validate active and GC'ed mapped-buffer count
  in cleanAllSlices() via close() ..

- Fix missing unmapping last buffer in notifyLengthChangeImpl(),
  branch criteria was off by one.

- cleanSlice(..) now also issues cleanBuffer(..) on the GC'ed entry,
  hence if WeakReference is still alive, enforce it's release.

- cleanBuffer(..) reverts FLUSH_PRE_HARD -> FLUSH_PRE_SOFT
  in case of an error.

- flush() -> flush(boolean metaData) to expose FileChannel.force(metaData).

- Add synchronous mode, flushing/syncing the mapped buffers when
  in READ_WRITE mapping mode and issue FileChannel.force() if not READ_ONLY.

  Above is implemented via flush()/flushImpl(..) for buffers and FileChannel,
  as well as in syncSlice(..) for buffers only.

  flush*()/syncSlice() is covered by:
    - setLength()
    - notifyLengthChange*(..)
    - nextSlice()

  Always issue flushImpl() in close().

- Windows: Clean all buffers in setLength(),
  otherwise Windows will report:

- Windows: Catch MappedByteBuffer.force() IOException

- Optimization of position(..)
  position(..) is now standalone to allow issuing flushSlice(..)
  before gathering the new mapped buffer.
  This shall avoid one extra cache miss.

  Hence rename positionImpl(..) -> position2(..).

- All MappedByteBufferOutputStream.write(..) methods
  issue syncSlice(..) on the last written current slice
  to ensure new 'synchronous' mode is honored.

+++

Unit tests:

- Ensure test files are being deleted

- TestByteBufferCopyStream: Reduced test file size to more sensible values.

-
Make Scripts OSX: Remove -java7 script, add -java6 script (default uses java8)