switch to go binary

[deitch]

This is a fundamental rewrite of the entire mysql-backup, several years in the making.

• the program running backup and restore is a single go-based binary, rather than a complex of scripts
• you can download and run the binary on its own, or from within a docker container (as before)
• most (but not all) of previously supported env vars continue to work
• this release will switch to the v1.0.0 version when ready; as of this PR, it will be v1.0.0-rc1
• latest tag will point to the most recent semver, not including -alpha*, -rc*, etc., i.e. only clean semver releases of format vX.Y.Z