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/models.rs

@@ -52,8 +52,10 @@ pub struct InterpretedWalRecord {
     /// A pre-serialized batch along with the required metadata for ingestion
     /// by the pageserver
     pub batch: SerializedValueBatch,
-    /// Byte offset within WAL for the end of the original PG WAL record
-    pub end_lsn: Lsn,
+    /// Byte offset within WAL for the start of the next PG WAL record.
+    /// Usually this is the end LSN of the current record, but in case of
+    /// XLOG SWITCH records it will be within the next segment.
+    pub next_record_lsn: Lsn,
     /// Whether to flush all uncommitted modifications to the storage engine
     /// before ingesting this record. This is currently only used for legacy PG
     /// database creations which read pages from a template database. Such WAL