Separate fatal.log from error.log

[fuzhe1989]

Enhancement

Fatal erros are that will cause process terminated. Fatal logs are used for outputing information of fatal errors.

Now TiFlash only has one kind of fatal logs: BaseDaemon will print logs when catch some signals and then exit. In the future we may have more kinds of fatal logs.

Fatal logs are different from normal error logs in that:

1. They are more important and happen rarely, so fsync could be affordable.
2. They could have different formats, e.g. do not escape \n, so that BaseDaemon can print the stacktrace in one log record.

[fuzhe1989]

Now the error.log already sets flush = true. This ensures we won't lose any error log, while also hurts the performance, since errors are not so rare.

[SchrodingerZhu]

Now the error.log already sets flush = true. This ensures we won't lose any error log, while also hurts the performance, since errors are not so rare.

Does this ensure alloc free？

[fuzhe1989]

@SchrodingerZhu IMO alloc free is an enhancement. Currently BaseDaemon will direclty use LOG_ERROR.