optimization of write-to/read-from mutation log file may cause problem

[qinzuoyan]

// write-to/read-from mutation log file, for better performance
//
// TODO(qinzuoyan): the optimization is to marshall code as int but not string,
// but this may cause problem, because log may be used by different program and
// the code map may change:
//   - the private log may be transfered to other node with different program
//   - the private/shared log may be replayed by different program when server restart
void write_to_log_file(std::function<void(const blob&)> inserter) const;
void write_to_log_file(binary_writer& writer) const;
static mutation_ptr read_from_log_file(binary_reader& reader, dsn_message_t from);