[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)
		}
	}