Improve log retention

[knolleary]

We currently retain logs in a fixed sized buffer within the launcher. This means:

1. the duration of retained logs depends on how noisy Node-RED is
2. the logs are only available if the launcher is running - and any history is lost if the launcher itself is restarted (eg suspend/resume).

This high level issue is to identify a plan for improving the log retention. We can make use of the fact we now have persistent file storage available to the instances.

Proposal: Write logs to the persistent file system on the instance.

• The file should be rotated daily
• Only retain the last N days of logs (14/30 days... could be configurable in the future)
• Provide API in the launcher to retrieve logs for a given time period - which can be restored from the log files (this will be a separate task needed by Time-based browsing of Instance logs flowfuse#5301 which I'll raise independently of this proposal.)
• Log files will show up under the 'assets' view of the instance - a feature not a bug, so should be under a well-defined directory

[hardillb]

Would need to be gated on persistent storge being enabled, but I can't see any show stoppers off the top of my head, will think about it some more.

Only problem with daily files is they could get very large, so would need to be careful serving them back to the forge app to send to the UI.

[hardillb]

Currently have files written to logs with 7 days of history

Logs are a JSON object per entry on a new line for each, this way we don't have to worry about closing out an array when the file is closed.

I have an API that will return a days worth of logs when passed a timestamp inside that day.

#328