public static byte[] add(final byte[] a, final byte[] b) {
        return add(a, b, EMPTY_BYTE_ARRAY);
    }
Concatenate byte arrays.

@param a lower half
@param b upper half
@return New array that has a in lower half and b in upper half.
stroom-bytebuffer/src/main/java/stroom/bytebuffer/hbase/Bytes.java