[3pt] Logging configuration

[Onvember]

Related dev. issue(s): tarantool/tarantool#689

Product: Tarantool
Root document:

• https://www.tarantool.io/en/doc/latest/reference/configuration/#logging
• https://www.tarantool.io/en/doc/latest/reference/reference_lua/log/

SME: @ gumix

Details

To configure log module eary without initializing box module at all, call the log.cfg({}) method, where the following
arguments are acceptable:

• log to specify file, pipe or syslog (same as
  box.cfg{log} option); the default value is nil
  and stderr is used;

• level to specify logging level; can be numbers (1-7) or
  strings (fatal,syserror,error,crit,warn,info,verbose,debug);
  the default value is 5 (i.e. 'info');

  While for symbolic representations of levels we allow only
  predefined keywords to be used, to keep backward compatibility
  the numbers can be any;

• format to specify output encoding ('plain' or 'json');
  the default value is 'plain';

• nonblock to open logging output in nonblocking mode
  (true or false); the default value is false.

Requested by @ cyrillos in tarantool/tarantool@a94a9b3.