Support rotation of the log file

[vladem]

Tell us more about this new feature.

When --log-directory CLI flag is provided Mountpoint will write its logs to a file in the specified directory. Mountpoint will open the file once and use obtained file handle forever. If the original file was renamed, logs will still go to this file. If the file was deleted, logs will be lost.

Currently, it's possible to rotate the log file by truncating it, e.g. by using copytruncate option of logrotate. There is a small risk of not recording logs arriving during this operation.

This feature request is to add support of log rotation to Mountpoint, which will ensure that no data is lost. There are a couple of options of doing that, namely:

1. just re-open the log file on SIGHUP (delegating the actual rotation to external tools);
2. rotation performed by Mountpoint with a configurable policy (frequency, max log file size).

We'll likely want to ensure that our implementation is compatible with cloudwatch-agent.