Add options to control caching of schemas

[justin-tay]

Closes #1016

This adds additional configuration options to control the caching / preloading schema behavior.

• Option to not cache schema $ref. This means that after resolving a $ref during evaluation the schema loaded is discarded and won't be used for subsequent evaluation runs. This is set to a default of true to cache by default. This may be needed if there is a large permutation of evaluation paths due to multiple nested applicators like allOf, oneOf and anyOf which makes caching all evaluation paths take up too much memory.
• Option to limit the max nesting depth when preloading $ref. This is set to a reasonable default of 40.

[txshtkckr]

Any chance we could get a release with this option in it? I'm encountering this issue in production use: https://bitbucket.org/atlassian/adf-builder-java/issues/105

On a related note, looking at CachedSupplier, it holds on to its delegate indefinitely. I have not dug into the library enough to be sure if there are thread-safety concerns around making the delegate mutable and null-ing it out after using it, but that would allow it to release all the references captured in the lambda at the call point, which could greatly reduce the memory pressure that this is causing.

In a heap dump that I analyzed, the retained set from the RefValidator was 450M, and even a very sprawled eval path tree shouldn't have this kind of impact. But the supplier captures the validation context, among possibly other things, and the heap I'm looking at shows it having captured 90,000 of those. The impact of that is almost definitely not trivial.

[txshtkckr]

If thread-safety is an issue, an easy workaround would be to change the delegate field to volatile and write back a constant supplier after performing the calculation:

    @Override
    public T get() {
        if (cache == null) {
            T value = delegate.get();
            cache = value;
            delegate = () -> value;
        }
        return cache;
    }

This would release whatever garbage the supplier is holding while still providing the guarantee that the delegate is never null.