MapOutputBuffer should use native byte order for kvmeta

I don't have a benchmark to support this, but this should give a small CPU improvement on the map output buffer: currently, we create kvmeta as ByteBuffer.wrap(kvbuffer).asIntBuffer(). According to the javadocs, the resulting int buffer will inherit its byte order from the ByteBuffer it comes from, and the byte buffer defaults to BIG_ENDIAN. Thus, all of our int access to/from the buffer will require byte-swapping.