Persist MPP resolution status in wallet file.

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

tests/test_lnpeer.py

@@ -316,6 +316,7 @@ class MockLNWallet(Logger, EventListener, NetworkRetryManager[LNPeerAddr]):
     maybe_cleanup_forwarding = LNWallet.maybe_cleanup_forwarding
     current_target_feerate_per_kw = LNWallet.current_target_feerate_per_kw
     current_low_feerate_per_kw = LNWallet.current_low_feerate_per_kw
+    maybe_cleanup_mpp = LNWallet.maybe_cleanup_mpp
 
 
 class MockTransport:
@@ -1741,6 +1742,7 @@ class TestPeerForwarding(TestPeer):
         ):
             alice_w = graph.workers['alice']
             bob_w = graph.workers['bob']
+            carol_w = graph.workers['carol']
             dave_w = graph.workers['dave']
             if mpp_invoice:
                 dave_w.features |= LnFeatures.BASIC_MPP_OPT
@@ -1762,6 +1764,12 @@ class TestPeerForwarding(TestPeer):
                 await asyncio.sleep(2)
             if result:
                 self.assertEqual(PR_PAID, dave_w.get_payment_status(lnaddr.paymenthash))
+                # check mpp is cleaned up
+                async with OldTaskGroup() as g:
+                    for peer in peers:
+                        await g.spawn(peer.wait_one_htlc_switch_iteration())
+                for peer in peers:
+                    self.assertEqual(len(peer.lnworker.received_mpp_htlcs), 0)
                 raise PaymentDone()
             elif len(log) == 1 and log[0].failure_msg.code == OnionFailureCode.MPP_TIMEOUT:
                 raise PaymentTimeout()