User activity logs

[achimnol]

Some customers want to keep detailed command logs executed by their users due to their security audit policies. Let's add an option to store all query-mode and batch-mode code snippets.

1st phase:

• [ ] Add an etcd configuration option
• [ ] Add the activity_logs database table
• [ ] Add an admin API to retrieve activity logs

2nd phase:

• [ ] Extend jail to log system call arguments and return values
  • This will incur non-negligible performance overheads. We should implement this very efficiently, and buffer & compress the logs on the fly as hundreds of syscalls may be issued per second.

[Yaminyam]

https://github.com/lablup/backend.ai/pull/1769 The created_user field has been deleted from that pr. This field existed to record cases where the created_user and the owner user of the session are different, and it was decided to record created_user as an event in the audit_log.