Method Put

Add a category to the cache, with the given ordinal as the value.

If the implementation keeps only a partial cache (e.g., an LRU cache) and finds that its cache is full, it should clear up part of the cache and return true. Otherwise, it should return false.

The reason why the caller needs to know if part of the cache was cleared is that in that case it will have to commit its on-disk index (so that all the latest category additions can be searched on disk, if we can't rely on the cache to contain them).

Ordinals should be non-negative. Currently there is no defined way to specify that a cache should remember a category does NOT exist. It doesn't really matter, because normally the next thing we do after finding that a category does not exist is to add it.