Add clearStream API in SystemAdmin

For use cases like batch processing, samza need to re-process from beginning for each run. This also requires the ability to reset internal metadata streams so the processors will not accidentally read the meta data from previous runs, e.g. checkpoints. In order to support this use case, we need a new API in SystemAdmin to clear streams.

For Kafka, clearStream will delete the topic in the brokers. This requires the following config for the brokers:

delete.topic.enable=true