This PR modularizes the configuration options of the query result cache
(QRC).
1. Add a feature toggle "allow_experimental_query_result_cache = false"
which controls whether the experimental QRC can be used at all. This
configuration option will be removed once the QRC becomes
non-experimental.
2. Modularize the existing two configuration settings
"enable_experimental_query_result_cache" and
"enable_experimental_query_result_cache_passive_usage" into three
orthogonal settings:
- "use_query_result_cache = false" controls whether a query (or all
queries of the session) utilize the query result cache
- settings "use_query_result_cache_passive_usage = true" and
"use_query_result_cache_active_usage = true" control the way a
query (or all queries of the session) utilize the query result
cache: "passive" use is reading, "active" use is writing.
