sc-state-db: Keep track of `LAST_PRUNED` after warp syncing (#2228)

When warp syncing we import the target block with all its state.
However, we didn't store the `LAST_PRUNED` block which would then lead
to `pruning` to forget about the imported block after a restart of the
node. We just set `LAST_PRUNED` to the parent block of the warp sync
target block to fix this issue.