Speed up PoW verification hashing

[reardencode]

In a microbenchmark, this method of reversing hex bytes is an order of magnitude faster. (100 nanoseconds vs. 1 microsecond for 10 bytes on an Intel Core i7-5600U)

[SamouraiDev]

There's Utils.reverseBytes() in bitcoinj. Can get rid of the member function altogether.

[reardencode]

Utils.reverseBytes is just for byte[], actually not sure why it exists since there's Arrays.reverse that is used later in calcHash. This method could also be modified to build up a byte[] instead of first building a String and then decoding it (I'm happy to do that if its your preference).

[SamouraiDev]

The use of String is mainly due to reading in from JSON and then logging for debug purposes. Better to keep them around for now.

[reardencode]

That was my thinking too.