thread-safe OntModel

[sszuev]

concurrent RDF-Graph (https://github.com/sszuev/concurrent-rdf-graph) is not enough.
OntModel methods are still not protected properly.

There is already concurrent implementation (com.github.owlcs.ontapi.OntologyModelImpl.Concurrent.asGraphModel), but it does not cover all cases.

TODO:

• Need to protect all write operations and all complex read-operations (such as Model#write, Model#write)
• Simple read operations should not be protected in performance sake (there is already protection for Iterator on graph level with internal own lock)
• The same protection should be done for OntObjects as well

[sszuev]

No one seems to be using this feature, hence low priority