Add log rotation and switch to flexi_logger

[pscott]

This PR adds the option to rotate logs. It does so by switching to another crate for loggin (previously env_logger, now flexi_logger).

A similar PR was already merged and reverted.

I would ask the reviewers to:

1. Thoroughly read the original issue.
2. Test and play a little bit with the configurations (using --log, --log-directory, --log-age, and --log-size). (cc @ddorgan )

This PR also fixes the problem of:

• AgeOrSize: if --log-age AND --log-size are specified, the log rotation will happen once either the AGE or the SIZE gets reached.
• Quitting if the Logger error'd: previously we would simply error and not start the client. Now the client simply prints an error message.

Closes #5926
polkadot companion: paritytech/polkadot#1450

[pscott]

Please note that logger.rs was added by #6564 but not properly reverted by #6627 because of #6620 .. :p

So I would add the reviewers to make sure the code is sound in logger.rs too :)

[ddorgan]

@pscott what happened to doing it the standard linux way? e.g. a SIGUP flushes and logrotate rotates.

[pscott]

@pscott what happened to doing it the standard linux way? e.g. a SIGUP flushes and logrotate rotates.

Since this comment said:

Though custom log rotation would still be useful I think. Not all platforms have journald and for and end users it might be easier to use something build-in.

I thought using flexi_logger with custom log rotation was the way to go. I think there's no clear consensus atm (see original issue), so this PR is here because the work was already done, but we could as well decide not to merge it.

If I understand @ddorgan you'd want SIGUP to flush and rotate? No custom "rotation" provided out of the box?

PS: stupid question but how would this work on windows? (I have no idea how logs are usually handled on windows...)

[pscott]

Since init_logger now returns a Result and requires an extra parameter, I tagged breaksapi. Feel free to remove :)

[gnunicorn]

closing because of inactivity.