Support JSON log format

[akiym]

{
  "time": 0,
  "tag": "workermamager",
  "action": "complete",
  "funcname": "Foo::Bar",
  "message": "...",
  "process": 0,
  "delay": 0,
  "insert_time": 0,
  "run_after": 0,
  "grabbed_until": 0,
  "failures": 0,
  "max_retries": 0
}