wal_decoder: rename end_lsn to next_record_lsn (#9776)

## Problem

It turns out that `WalStreamDecoder::poll_decode` returns the start LSN
of the next record and not the end LSN of the current record. They are
not always equal. For example, they're not equal when the record in
question is an XLOG SWITCH record.

## Summary of changes

Rename things to reflect that.

libs/wal_decoder/src/serialized_batch.rs

@@ -137,7 +137,7 @@ impl SerializedValueBatch {
     pub(crate) fn from_decoded_filtered(
         decoded: DecodedWALRecord,
         shard: &ShardIdentity,
-        record_end_lsn: Lsn,
+        next_record_lsn: Lsn,
         pg_version: u32,
     ) -> anyhow::Result<SerializedValueBatch> {
         // First determine how big the buffer needs to be and allocate it up-front.
@@ -161,13 +161,17 @@ impl SerializedValueBatch {
             let key = rel_block_to_key(rel, blk.blkno);
 
             if !key.is_valid_key_on_write_path() {
-                anyhow::bail!("Unsupported key decoded at LSN {}: {}", record_end_lsn, key);
+                anyhow::bail!(
+                    "Unsupported key decoded at LSN {}: {}",
+                    next_record_lsn,
+                    key
+                );
             }
 
             let key_is_local = shard.is_key_local(&key);
 
             tracing::debug!(
-                lsn=%record_end_lsn,
+                lsn=%next_record_lsn,
                 key=%key,
                 "ingest: shard decision {}",
                 if !key_is_local { "drop" } else { "keep" },
@@ -179,7 +183,7 @@ impl SerializedValueBatch {
                     // its blkno in case it implicitly extends a relation.
                     metadata.push(ValueMeta::Observed(ObservedValueMeta {
                         key: key.to_compact(),
-                        lsn: record_end_lsn,
+                        lsn: next_record_lsn,
                     }))
                 }
 
@@ -210,7 +214,7 @@ impl SerializedValueBatch {
                 // that would corrupt the page.
                 //
                 if !page_is_new(&image) {
-                    page_set_lsn(&mut image, record_end_lsn)
+                    page_set_lsn(&mut image, next_record_lsn)
                 }
                 assert_eq!(image.len(), BLCKSZ as usize);
 
@@ -229,12 +233,12 @@ impl SerializedValueBatch {
 
             metadata.push(ValueMeta::Serialized(SerializedValueMeta {
                 key: key.to_compact(),
-                lsn: record_end_lsn,
+                lsn: next_record_lsn,
                 batch_offset: relative_off,
                 len: val_ser_size,
                 will_init: val.will_init(),
             }));
-            max_lsn = std::cmp::max(max_lsn, record_end_lsn);
+            max_lsn = std::cmp::max(max_lsn, next_record_lsn);
             len += 1;
         }