Parallel query: allow parallel scans on secondary indexes in InnoDB

While MDEV-39491 focuses on full table scans on the primary index, parallel scanning should be also available for scans on secondary indexes. The goal of this task is to:

• enable partitioning to chunks for full index scans on InnoDB secondary indexes;
• enable partitioning to chunks for range scans on InnoDB secondary indexes;
• let the optimizer run parallel execution for tables accessed by secondary indexes;
• measure performance impact of parallelism on those scans.