Events for all kinds of Flame

[k82cn]

Motivation

It's better to support events of all kind of Flame, e.g. app, ssn and tasks to improve the debugability.

Function Specification

Add a record_event function in storage model to persist events of all kinds.

Solutions

1. Add a record_event function in storage model
2. Add a events table in database; the colums are kind, id, creation_time and event.
3. Add a events manager to clean/merge similar events

Additional context

N/A