java_gen: Return copies of byte arrays in getBytes LOXI-58

The getBytes() methods of various classes in openflow.types were
returning and caching instances of byte[], which is mutable. They
now return clone()'d instances.
java_gen: use Arrays.copyOf() instead of .clone() for defensive copy

@gregor-bsn convinced me it's better :)
Merge into master from pull request #204:
java_gen: Return copies of byte arrays in getBytes LOXI-58 (https://github.com/floodlight/loxigen/pull/204)