add a concurrent 'stress test' for the knowledge base
add a concurrent 'stress test' for the knowledge base

10 threads writing 200 statements and generating 100 queries each,
and checking that reasoning is correctly performed.

Currently runs in 22.21s on 11th Gen Intel i7-1185G7 (8) @ 4.800G with
32GB RAM.

severin-lemaignan/minimalkb