Move quota checking code into a separate method

Instead of running `utils._get_dir_size(self.artifactdir)` to get the
size of the cache, it would be better to reuse
ArtifactCache.get_cache_size(), otherwise we would have to fully
calculate the size of the artifact cache twice at startup.

As the ArtifactCache is not yet instantiated when Context.load() runs,
move most of the quota checking code into a separate function which can
be called after the ArtifactCache is instantiated.