Let user choose compression program for backups

Description

The current backup process is painfully long thanks to the single-threaded gzip, especially when upgrading. I would like to bring gitlab-ce#20773 back into life because:

1. More and more cores are becoming the standard.
2. Parallel compression programs (like xz and pigz) allow users to specify thread number.
3. For on-premise instances, operators may schedule backup at low traffic times (e.g. midnight), so there's no worry about eating up all system resources.

Proposal

Have a configuration option to let users choose their preferred compression program (and arguments), defaulting to gzip.

Thanks for your consideration.