KVStore upstreaming followups

[tnull]

Closes #2561.

Addresses two last feedback items from #2472.

[valentinewallace]

Works ✅

[tnull]

Force-pushed including the following changes:

> git diff-tree -U2  9ea0d03f aeaed628
diff --git a/lightning/src/util/persist.rs b/lightning/src/util/persist.rs
index 7a73aba0..372a094a 100644
--- a/lightning/src/util/persist.rs
+++ b/lightning/src/util/persist.rs
@@ -180,5 +180,4 @@ impl<ChannelSigner: WriteableEcdsaChannelSigner, K: KVStore> Persist<ChannelSign
        fn persist_new_channel(&self, funding_txo: OutPoint, monitor: &ChannelMonitor<ChannelSigner>, _update_id: MonitorUpdateId) -> chain::ChannelMonitorUpdateStatus {
                let key = format!("{}_{}", funding_txo.txid.to_hex(), funding_txo.index);
-               debug_assert!(key.len() > 65);
                match self.write(
                        CHANNEL_MONITOR_PERSISTENCE_NAMESPACE,
@@ -193,5 +192,4 @@ impl<ChannelSigner: WriteableEcdsaChannelSigner, K: KVStore> Persist<ChannelSign
        fn update_persisted_channel(&self, funding_txo: OutPoint, _update: Option<&ChannelMonitorUpdate>, monitor: &ChannelMonitor<ChannelSigner>, _update_id: MonitorUpdateId) -> chain::ChannelMonitorUpdateStatus {
                let key = format!("{}_{}", funding_txo.txid.to_hex(), funding_txo.index);
-               debug_assert!(key.len() > 65);
                match self.write(
                        CHANNEL_MONITOR_PERSISTENCE_NAMESPACE,
@@ -219,6 +217,5 @@ where
                CHANNEL_MONITOR_PERSISTENCE_NAMESPACE, CHANNEL_MONITOR_PERSISTENCE_SUB_NAMESPACE)?
        {
-               debug_assert!(stored_key.len() > 65);
-               if stored_key.len() < 65 {
+               if stored_key.len() < 66 {
                        return Err(io::Error::new(
                                io::ErrorKind::InvalidData,