Rebuild pending payments list before replaying pending claims/fails

On `ChannelManager` reload we rebuild the pending outbound payments
list by looking for any missing payments in `ChannelMonitor`s.
However, in the same loop over `ChannelMonitor`s, we also re-claim
any pending payments which we see we have a payment preimage for.

If we send an MPP payment across different chanels, the result may
be that we'll iterate the loop, and in each iteration add a
pending payment with only one known path, then claim/fail it and
remove the pending apyment (at least for the claim case). This may
result in spurious extra events, or even both a `PaymentFailed` and
`PaymentSent` event on startup for the same payment.

Backport of 8106dbf

Resolved substantial conflcits in:
 * lightning/src/ln/channelmanager.rs by simply rewriting the
   patch.

Note that the `is_channel_closed` variable used in the upstream
version of this commit replaced simply checking if the
`outpoint_to_peer` map had an entry for the channel's funding
outpoint.

Note that the next upstream commit in this series
(3239d67) is unnecessary on this
branch as we use `outpoint_to_peer` rather than `per_peer_state` to
detect channel closure here.

Author: TheBlueMatt

lightning/src/ln/channelmanager.rs

@@ -13869,6 +13869,9 @@ where
 			// payments which are still in-flight via their on-chain state.
 			// We only rebuild the pending payments map if we were most recently serialized by
 			// 0.0.102+
+			//
+			// First we rebuild all pending payments, then separately re-claim and re-fail pending
+			// payments. This avoids edge-cases around MPP payments resulting in redundant actions.
 			for (_, monitor) in args.channel_monitors.iter() {
 				let counterparty_opt = outpoint_to_peer.get(&monitor.get_funding_txo().0);
 				if counterparty_opt.is_none() {
@@ -13887,6 +13890,11 @@ where
 							);
 						}
 					}
+				}
+			}
+			for (_, monitor) in args.channel_monitors.iter() {
+				let counterparty_opt = outpoint_to_peer.get(&monitor.get_funding_txo().0);
+				if counterparty_opt.is_none() {
 					for (htlc_source, (htlc, preimage_opt)) in monitor.get_all_current_outbound_htlcs() {
 						let logger = WithChannelMonitor::from(&args.logger, monitor, Some(htlc.payment_hash));
 						match htlc_source {