Optimize the MultiByteBuff#get(ByteBuffer, offset, len)

In HBASE-22483,  we saw that the BucketCacheWriter thread was quite busy [^BucketCacheWriter-is-busy.png],  the flame graph also indicated that the ByteBufferArray#internalTransfer cost ~6% CPU (see async-prof-pid-25042-cpu-1.svg). because we used the hbase.ipc.server.allocator.buffer.size=64KB, each HFileBlock will be backend  by a MultiByteBuff: one 64KB offheap ByteBuffer and one small heap ByteBuffer.   

The path is depending on the MultiByteBuff#get(ByteBuffer, offset, len) now: 

RAMQueueEntry#writeToCache
    |--> ByteBufferIOEngine#write
        |--> ByteBufferArray#internalTransfer
            |--> ByteBufferArray$WRITER
                |--> MultiByteBuff#get(ByteBuffer, offset, len)

While the MultiByteBuff#get impl is simple and crude now, can optimze this implementation:

  @Override
  public void get(ByteBuffer out, int sourceOffset,
      int length) {
    checkRefCount();
      // Not used from real read path actually. So not going with
      // optimization
    for (int i = 0; i < length; ++i) {
      out.put(this.get(sourceOffset + i));
    }
  }