rgw/lua: create fresh VM for each background script execution

rgw/lua: create fresh VM for each background script execution

Previously, the background thread reused the same Lua VM across
iterations, causing stale state to persist. This made operations
like 'pairs(RGW)' fail to iterate properly.

Now we create a fresh VM on each iteration, which:
- Fixes the iteration bug
- Simplifies the code (no need to update limits on existing VM)
- Ensures clean state for each script execution

Verified with unit tests:
- TableIterateBackground
- TableIterateBackgroundBreak
- TableIterateStepByStep

Fixes: https://tracker.ceph.com/issues/74839
Signed-off-by: Rotem Shapira <rotem.rs@gmail.com>