parser.getName is ambiguous for RioParserImpl, use .toString instead
change PriorityCollection to use a ConcurrentSkipListSet internally

There are no alternatives to PriorityCollection for standard use of the
API, so it must be threadsafe.
Ignore JSONLD for now as it is actually quite broken
make rio parsers not fail on datatype and language tag verification
make some more tests pass in maven by running them in separate threads
Use isEmpty instead of .size for efficiency
Add two duplicate tests to try to replicate the behaviour, but to no end