Improve logging by splitting into multiple streams for easier cross-section analysis

To avoid too high volume of debug logs, currently Backend.AI Manager and Agent provides `[debug].log-xxxx` boolean options to enable/disable specific type of log messages, as shown in:https://github.com/lablup/backend.ai/blob/833ed5477d57846e568b17fec35c82300111a519/configs/agent/sample.toml#L293-L331

However, this approach makes it harder to perform a postmortem anaylsis on customer sites because we usually turn off many of these log "sections" by default.

Let's split out them to multiple different log streams with higher log levels. For example, as most container engine events are logged in the DEBUG level with `log-docker-events = true` currently, let's promote them to the INFO level using a separate "agent-container-events.log" output stream.

This will allow easier cross-section, postmortem analysis if combined with additional log viewer tools like #1138.

JIRA Issue: BA-191