DL-206: Delete the log should also delete the underline ledgers

[yzang]

Problem:
We're not deleting the ledgers when we delete the whole log stream using dlm.delete() API. This would cause a lot of garbage/orphan ledgers in Bookkeeper.

The fix is to delete the ledger when we delete the log stream. Also added a test to validate.

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[sijie]

@yzang awesome! this change looks good!