Include saved mtime and fingerprint when serializing buffers

This still doesn't include:

- An assertion in the randomized test to ensure buffers are not spuriously
marked as modified
- Sending an update when synchronizing buffers after a reconnection

crates/rpc/proto/zed.proto

@@ -975,6 +975,8 @@ message BufferState {
     string base_text = 3;
     optional string diff_base = 4;
     LineEnding line_ending = 5;
+    string saved_version_fingerprint = 6;
+    Timestamp saved_mtime = 7;
 }
 
 message BufferChunk {