Consider replacing cron with supercronic

[LeoAdamek]

As per Fly's docs -- Standard system cron daemons aren't very suited to container-based environments. Primarily because they clear the env to prevent disseminating secrets.

However this means that some barman features just don't quite work right, such as using barman-cloud-wal-archive as a pre-archive script hook to automatically relay WALs to external storage. It'd be great if supercronic shipped in the image for this purpose, and the start command would use this instead.

Currently my workaround is to add the pre_archive script to my barman.conf manually and manually download and run supercronic via nohup so as not to need modification to the container image.

If a PR is wanted, I could look at it next week I think.

[davissp14]

@LeoAdamek Just an fyi, we are looking to deprecate the separate barman machine in favor of running archiving next to the primary, which should simplify on-going management.

We haven't cut a release for this yet, but some details can be found here: #231