Fix StartableMethods.stopSequentially

- need to mark sub-tasks as inessential; otherwise failure of one
  will cause all subsequent tasks not to be executed!
Fix MySqlNode feed termination

- was not stopping feed (and was doing unnecessary extra ssh
  because was using separate SshFeed instance for each sensor)
stopSequentially: incorporate review comments
Merge pull request #1306 from aledsage/fix/stopSequentially

Fix StartableMethods.stopSequentially