Change the default maximum OP_RETURN size to 80 bytes

The maximum size for OP_RETURN outputs used to be 80 bytes, then got changed to 40 bytes to be on the safe side. We have now been running with 40 bytes for about 9 months, and nothing catastrophic happened to the Blockchain, so I am proposing to increase it back to 80 bytes.

Also, that value is now configurable through the `datacarriersize` option, so miners who want to stay on 40 bytes (or any other value) can easily do so.