Release v2.0-alpha

Changelog:

1. Bi-directional stream API is used, allowing servers to issue
   diagnosing telemetry commands to clients;
2. Server side stream API is employed for receive message API, which
   mitigates costs of serialization/deserialization of large data chunks
3. Error codes are re-organized, roughly following HTTP semantics. Each
   code now has five digits, the first three of them complies HTTP status
   code; The rest two digits are used to specify each exact scenario.
4. A few concepts are renamed to follow RocketMQ 5.x terminologies.