[canal] add a skip prepareDumper() logic

[zhujintao]

exec: "mysqldump": executable file not found in $PATH

	if err = c.prepareDumper(); err != nil {
		return nil, errors.Trace(err)
	}

use custom full import, non mysqldump
add skipDump logic:

	if !c.cfg.skipDump {
		if err = c.prepareDumper(); err != nil {
			return nil, errors.Trace(err)
		}
	}

[rockwotj]

Yeah it would be cool to replicate what mysqldump does in go instead of shelling out

[dveeden]

I think adding a skipDump option could be useful. Replacing mysqldump/mariadb-dump would be welcome, but it is also a lot of work to get that working correctly.