Optimise gas use for `SettleDealPaymentsExported`

[rjan90]

(Follow-up from filecoin-project/FIPs#1224 and filecoin-project/lotus#13471; this ticket is specifically about reducing the builtin-actors execution cost, while the Lotus issue tracks client-side gas-aware batching.)

We have seen SettleDealPaymentsExported messages large enough to consume a significant fraction of a block on their own, and they are expected to become more common as deal settlement tooling gets used more heavily.

Some examples:

• bafy2bzaceargwm7arrllv3vo62qloq7muchbavlle3en25zisb3h4cli5p2ca: SettleDealPaymentsExported, gas limit 6.41B, gas used 5.13B, included in 5 blocks.
• bafy2bzaceadslcftv6z4wxamz5puspsx2mreec6v77uc2gdwzpozpfyffjcgy: SettleDealPaymentsExported, gas limit 8.78B, gas used 7.02B, included in 2 blocks.
• bafy2bzaceb2ebhdrfzawonxfixob6kdro53e3g4hulhosumxjoymjfzosbfjq: SettleDealPaymentsExported, gas limit 6.89B, gas used 5.51B, included in 4 blocks.

The actor path is:

• settle_deal_payments loops over each deal ID here.
• Each deal loads the proposal/state, serializes/hashes the proposal to a deal CID, calls get_active_deal_or_process_timeout, and then calls process_deal_update here.
• process_deal_update transfers elapsed payment per deal here, and transfer_balance reloads and flushes the balance tables through unlock_balance here.
• The final deal-state and sector mapping writes are already batched through put_deal_states and remove_sector_deal_ids here and here, so the obvious investigation area is the repeated per-deal accounting / table load-flush work before those final batched writes.

Possible things to investigate:

• Batch or aggregate payment/accounting table updates by client/provider where the settlement semantics allow it.
• Avoid repeatedly loading/flushing the same escrow/locked balance tables inside a single settlement transaction.
• Look for per-deal work that can be moved outside the loop, or avoided for common cases.
• Re-check this after the cleanup tracked in Remove async handling of terminated deals from market actor #1388, since removing slashed-deal compatibility paths may simplify process_deal_update.

This is similar in spirit to #1610: reducing the execution cost of high-fanout messages rather than relying only on senders to choose smaller batches.