Move initializing process for @query_cache to QueryCache module.

There are all operation about `@query_cache` / `@query_cache_enabled` instance variables in `QueryCache` module.
But only initializing process for them in `initialize` of `AbstractAdapter`.

In order to separate the concern, I think that initializing process also should be moved to `QueryCache` module.
When reading code, I can't find initializing process easily.