Only the latest log was kept as `node.log.old`. Now, log files are
numbered (`node.log.1`, `node.log.2` and so on). Also `node.log` is now
a hard link to the current log file. `rad node stop` will delete
`node.log` (note that `node.log.x` stays intact) and `rad node start`
will delete `node.log` before it creates `node.log.y` in case the node
crashed.
Also, when running in foreground mode, now a log file is created. It
just contains the hint that the node was started in foreground mode,
just to avoid confusion.
The implementation is split into a sans I/O part as `struct LogRotator`,
while the I/O counterpart is captured by `struct LogRotatorFileSystem`.
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