This adds functionality so that it's possible to use `Backup` and
`BackupShard` and enforce that the backup created is safe to be used for
upgrading MySQL.

Signed-off-by: Dirkjan Bussink <d.bussink@gmail.com>

Signed-off-by: Dirkjan Bussink <d.bussink@gmail.com>

This adds the version number and upgrade safe state to the manifest. If
these values are set, they are also validated on restore.

For backwards compability we always allow still empty version numbers.

Signed-off-by: Dirkjan Bussink <d.bussink@gmail.com>

This already happens correctly with the shutdown for the builtin engine,
where we're shutdown twice for `vtbackup` then. With this move we also
properly honor the `UpgradeSafe` flag if the path is called through
`Backup` or `BackupShard` vtctld commands.

Signed-off-by: Dirkjan Bussink <d.bussink@gmail.com>

It's not guaranteed that MySQL will always be up when doing a restore,
this is shown in some of the failing tests with the changes so far.

This means we need a way to get the MySQL version without having it
running. We can though rely in `mysqlctld` being up. So this adds a new
RPC to retrieve the version number through invoking the binary. This
ensures we still get the right version also if we're running from the
tablet as we'd execute the RPC.

We also have less of a versioning issue here, as the vttablet + mysql
are always upgraded together and the new mysqlctl is injected into the
mysql container if needed so we don't have a version mismatch. So it's
safe to do this in one step.

Also decided to make the upgrade safe an explicit flag for `vtbackup` as
well since overloading the existing flag was wonky and not really what
we'd want. This makes it more explicit and safe.

Signed-off-by: Dirkjan Bussink <d.bussink@gmail.com>

Signed-off-by: Dirkjan Bussink <d.bussink@gmail.com>