public String toString() {
        return "GrowableByteBuffer"
                + "[pos="+ position()
                + " lim=" + limit()
                + " cap=" + capacity()
                + " grow=" + growFactor
                + "]";
    }
Behaves as ByteBuffer slicing, but the internal buffer will no longer be
shared if one of the buffers is forced to grow.

@return a new buffer with shared contents
@see ByteBuffer#slice()
vespajlib/src/main/java/com/yahoo/io/GrowableByteBuffer.java