Previously HOST1 defined what it would run for HOST2 (FALLBACK_HOST1_COVERS_HOST2_TIER*).
Now each host defines what it wants run when IT goes down (FALLBACK_HOST1_TIER*), and the
covering server reads the down host's conf via the RAM/persistent cache.
get_tier_containers() reads FALLBACK_${REMOTE_ID}_TIER* instead of
FALLBACK_${MY_ID}_COVERS_${REMOTE_ID}_TIER*. Tier data migrated to the correct host confs.
Writeback paths and delays were already REMOTE_ID-based — no change needed there.