Use oldest_block_slot to break of pruning payloads

[dapplion]

Issue Addressed

Lighthouse DB persists an "anchor" which has some useful data. One is the anchor_slot. Tree-states on the hot DB wants to use that value and change it's meaning slightly. The only blocker for that change is try_prune_execution_payloads which uses the anchor_slot as optimization to break of early the pruning routine.

BUT it's an easy fix. From @michaelsproul

I was thinking maybe we could recycle the anchor_slot for use here, as it is fairly useless and not used for much. The only load-bearing use of the anchor_slot that I can find is in try_prune_execution_payloads where it is used to halt the pruning process. However, this is not necessary as we could either:

1. Keep iterating back to Bellatrix, or
2. Stop iterating back once we find a payload that is missing, or
3. Use the oldest_block_slot to determine when to stop iterating back. This is my preferred option, as it is also compatible with storing execution payloads older than the anchor_slot, i.e. Store execution payloads during backfill if --prune-payloads false #6510

ref dapplion#39 (comment)

Proposed Changes

This PR implements option 3 of the above citation, quite painless