Persist MPP resolution status in wallet file.

[ecdsa]

If we accept a MPP and we forward the payment (trampoline or swap), we need to persist the payment accepted status, or we might wrongly release htlcs on the next restart.

lnworker.received_mpp_htlcs used to be cleaned up in maybe_cleanup_forwarding, which only applies to forwarded payments. However, since we now persist this dict, we need to clean it up also in the case of payments received by us. This part of maybe_cleanup_forwarding has been migrated to lnworker.maybe_cleanup_mpp