The VDBE step() function was taking Arc<MvStore> by value, causing it to
be cloned on every single step of query execution. This resulted in
thousands of atomic reference count increments/decrements per query,
showing up as a major hotspot in profiling.
Changed step() and related functions to take Option<&Arc<MvStore>>
instead, passing a reference rather than cloning the Arc. This eliminates
the unnecessary atomic operations while maintaining the same semantics.
4c5a7cda08