impr(pageserver): error if keys are unordered in merge iter (#8818)

In case of corrupted delta layers, we can detect the corruption and bail
out the compaction.

## Summary of changes

* Detect wrong delta desc of key range 
* Detect unordered deltas

Signed-off-by: Alex Chi Z <chi@neon.tech>

pageserver/src/tenant/storage_layer/delta_layer.rs

@@ -232,6 +232,18 @@ pub struct DeltaLayerInner {
     max_vectored_read_bytes: Option<MaxVectoredReadBytes>,
 }
 
+impl DeltaLayerInner {
+    pub(crate) fn layer_dbg_info(&self) -> String {
+        format!(
+            "delta {}..{} {}..{}",
+            self.key_range().start,
+            self.key_range().end,
+            self.lsn_range().start,
+            self.lsn_range().end
+        )
+    }
+}
+
 impl std::fmt::Debug for DeltaLayerInner {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         f.debug_struct("DeltaLayerInner")
@@ -1527,6 +1539,10 @@ pub struct DeltaLayerIterator<'a> {
 }
 
 impl<'a> DeltaLayerIterator<'a> {
+    pub(crate) fn layer_dbg_info(&self) -> String {
+        self.delta_layer.layer_dbg_info()
+    }
+
     /// Retrieve a batch of key-value pairs into the iterator buffer.
     async fn next_batch(&mut self) -> anyhow::Result<()> {
         assert!(self.key_values_batch.is_empty());