`MergeRequests::Mergeability::RunChecksService#run_check` expects
that `MergeRequests::Mergeability::ResultsStore#read` to return
a `MergeRequests::Mergeability::CheckResult` object.

This fixes the `ResultsStore#read` to return a `CheckResult` object
once it received a result from cache (e.g. Redis).

This is behind the `mergeability_caching` feature flag.