scan: change backend_idx to backend address

[iwanbk]

Currently SCAN command use backend_idx to complement the cursor argument.
Cursor is needed for the SCAN to work, it is like which page we want to scan.
While backend_idx is the index of the metadata backend in the internal vector of the metastore.

Using backend_idx is not approriate because the index can changed when we refresh the metastore.

Scan is currently only used in two places:

• by metadata rebuild: it will not impacted by index change
• by data rebuild: it'll impacted, but will not create any damage.

We should fix it anyway to avoid future problem.