fix: cache invalidation under concurrent queries

The configuration cache is a part of the executor's runtime.
Previously, quech query was holding a immutable borrow of the
runtime, while cache invalidation tried to get a mutable borrow
(apply_config). As a result, the cache invalidation always failed
to mutably borrow the runtime.
Current commit fixes the issue by wrapping the configuration in
a reference cell. As a result, apply_config now immutably borrows
the runtime and don't fail on this step.