Hudi streaming-read is not stopping with savepoint correctly

Flink supports stopping with savepoint as documented here:

https://nightlies.apache.org/flink/flink-docs-master/docs/ops/state/savepoints/#stopping-a-job-with-savepoint

Stopping with savepoint will invoke these 3 interface functions Flink functions.

1. cancel()
2. snapshotState()
3. close()

However, the current implementation of stopping with savepoint will cause an issuedInstant to be null when snapshotState is invoked. This is so as cancel() will set the issuedInstant to null, causing the snapshotState() to add a null value to the LinkState list.

As such, when resuming from a savepoint, there will be:

1. data loss if the LATEST (by not specifying a value for, i.e. default value) read.start.commit is used
2. duplicated data if the EARLIEST read.start.commit is used