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

[achimnol]

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:

backend.ai/configs/agent/sample.toml

Lines 293 to 331 in 833ed54

	[debug]
	# Enable or disable the debug-level logging.
	enabled = false
	# If set true, it does not actually delete the containers after they terminate or are terminated
	# so that developers can inspect the container logs.
	# This is useful for debugging errors that make containers to terminate immediately after kernel
	# launches, due to bugs in initialization steps such as jail.
	skip-container-deletion = false
	# Enable or disable the asyncio debug mode.
	asyncio = false
	# Use the custom task factory to get more detailed asyncio task information; this may have performance penalties
	enhanced-aiomonitor-task-info = false
	# Enable the debug mode of the kernel-runner.
	kernel-runner = false
	# Include debug-level logs for internal events.
	log-events = false
	# Include debug-level logs for detailed kernel creation configs and their resource spec.
	log-kernel-config = false
	# Include debug-level logs for allocation maps.
	log-alloc-map = false
	# Include debug-level logs for statistics.
	log-stats = false
	# Include debug-level logs for heartbeats
	log-heartbeats = false
	# Set the interval of agent heartbeats in seconds.
	heartbeat-interval = 20.0
	# Include debug-level logs for docker event stream.
	log-docker-events = false

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.