Acquire per_peer_state write lock earlier in ChannelManager::write

Previously, we would hold the `per_peer_state` `read` lock during the
serialization of channel objects. While this might have reduced lock
congestion minimally, we also calculated the number of serializable
peers in this phase, only to use it later after dropping and reacquiring
the `per_peer_state` `write` lock.

This could potentially result in inconsistiencies if the
`serializable_peer_count` would change after we dropped the `read` lock
but before we acquired the `write` lock.

To mitigate the issue we just acquire the `write` lock ealier and hold
it for the remainder of `ChannelManager::write`.

Author: tnull

lightning/src/ln/channelmanager.rs

@@ -11035,9 +11035,10 @@ where
 			best_block.block_hash.write(writer)?;
 		}
 
+		let per_peer_state = self.per_peer_state.write().unwrap();
+
 		let mut serializable_peer_count: u64 = 0;
 		{
-			let per_peer_state = self.per_peer_state.read().unwrap();
 			let mut number_of_funded_channels = 0;
 			for (_, peer_state_mutex) in per_peer_state.iter() {
 				let mut peer_state_lock = peer_state_mutex.lock().unwrap();
@@ -11084,8 +11085,6 @@ where
 			decode_update_add_htlcs_opt = Some(decode_update_add_htlcs);
 		}
 
-		let per_peer_state = self.per_peer_state.write().unwrap();
-
 		let pending_inbound_payments = self.pending_inbound_payments.lock().unwrap();
 		let claimable_payments = self.claimable_payments.lock().unwrap();
 		let pending_outbound_payments = self.pending_outbound_payments.pending_outbound_payments.lock().unwrap();