Rollup node/Store: fix background store migration

What

This MR makes the store migration code not overwrite data that is more up to date.

Why

During the background migration of the store, the data (singleton values) that is in the store can become outdated wrt. to the evolving state of the rollup node. This has little impact on the rollup node itself because these will autocorrect, but because of this, the rollup node would send old heads in its stream to the EVM sequencer at the end of the migration.

Manually testing the MR

Checkout before the third commit and verify that the tests fail with

dune exec src/lib_smart_rollup_node/test/main.exe -- 'title =~ store_migration' -k

Run the same command on the head of this branch to see them succeed.