add timestamp field to segment header

Author: MarinPostma

libsql-wal/src/segment/compacted.rs

@@ -23,6 +23,7 @@ pub struct CompactedSegmentDataHeader {
     pub(crate) size_after: lu32,
     /// for now, always 4096
     pub(crate) page_size: lu16,
+    pub(crate) timestamp: lu64,
 }
 impl CompactedSegmentDataHeader {
     fn check(&self) -> Result<()> {

libsql-wal/src/segment/sealed.rs

@@ -94,6 +94,7 @@ where
             version: LIBSQL_WAL_VERSION.into(),
             magic: LIBSQL_MAGIC.into(),
             page_size: self.header().page_size,
+            timestamp: self.header.sealed_at_timestamp,
         };
 
         hasher.update(header.as_bytes());

libsql-wal/src/storage/compaction/mod.rs

@@ -246,6 +246,9 @@ impl<B> Compactor<B> {
             version: LIBSQL_WAL_VERSION.into(),
             magic: LIBSQL_MAGIC.into(),
             page_size: last_header.page_size,
+            // the new compacted segment inherit the last segment timestamp: it contains the same
+            // logical data.
+            timestamp: last_header.timestamp,
         };
 
         hasher.update(header.as_bytes());