Performance enhancement for larger API jsons

[Vity01]

At this time the whole check runs in a single thread. For larger API jsons (300+ paths) processing takes a while.
Simple math: 300paths * count of breaking changes checks. It does not scale much.
The processing can be launched in parallel
In DefaultBreakChecker
return rules.stream().map(rule -> rule.checkRule(oldApi, newApi)).flatMap(Collection::stream).collect(toList());

with just a replacement of stream() into parallelStream() it runs faster...
Unfortunately it has some possible side effects. The order of result list of breaking changes is unpredictable - that can be confusing for user. Therefore I also suggest to add a support for BreakingChange priority and to sort the result list by this value. Eg. PathRemoval breaking change can have higher order priority than others.

Also I am not sure about implementation of classes SeenRefHolder, SchemaStoreProvider which they are using ThreadLocal.

There are also other possible places for speed improvements. Eg. loading old and new api can be launched in parallel as well. With Project reactor it can be a piece of cake.