Update index & offset encoding

Previously, the index & offsets were encoded as 8-character hex
strings, however, this limits the maximum value to a `uint32`. This
is normally not an issue, however, indices could go over the maximum
value of 4 billion over time and the offset could exceed this value
for an especially large WAL update. For safety, these encodings have
been updated to 16-character hex encodings.

file_replica_client.go

@@ -100,7 +100,7 @@ func (c *FileReplicaClient) WALSegmentPath(generation string, index int, offset
 	if err != nil {
 		return "", err
 	}
-	return filepath.Join(dir, FormatIndex(index), fmt.Sprintf("%08x.wal.lz4", offset)), nil
+	return filepath.Join(dir, FormatIndex(index), fmt.Sprintf("%s.wal.lz4", FormatOffset(offset))), nil
 }
 
 // Generations returns a list of available generation names.