Fix restart bugs in ZkCloudnameIntegrationTest.

There were a few bugs related to restarting `EmbeddedZooKeeper` that caused `ZkCloudnameIntegrationTest` to fail on Windows:

- `EmbeddedZooKeeper.shutdown()` didn't close the transaction log, making deletion fail on Windows since the file handle was still open
- `EmbeddedZooKeeper.del()` didn't verify that files were successfully
  deleted
- `ZkCloudNameIntegrationTest.testZookeeperRestart()` didn't retry coordinate creation if the connection to ZooKeeper failed, leading to a race condition when restarting ZooKeeper, as coordinate creation doesn't wait for ZooKeeper to get back online

This commit fixes these bugs, allowing the unit test suite to run successfully on Windows.