Add more test cases on LongZkLedgerIdGenerator

[sijie]

#900 fix a bug in LongZkLedgerIdGenerator. However it is a bit difficult to test with a real zookeeper client. It would be good to think about add a test case with mock zookeeper client to cover as many cases as possible.

[merlimat]

@sijie in Pulsar we have a MockZookeeper class that can be used to trigger ZK failures at a specific point, eg: "fail after 3 steps" https://github.com/apache/incubator-pulsar/blob/f21cb67fcaccd4e77d47a2e1171cf5506dd8ec57/managed-ledger/src/test/java/org/apache/zookeeper/MockZooKeeper.java#L844

[sijie]

awesome