Watch key support in databend-meta

Databend-meta uses GRPC stream to watch the key changes, users can add a watch key through KVApi, and the WatchRequest parameters include:

• key: the key to register for watching.
• key_end: key_end is the end of the range [key, key_end) to watch. If key_end is None, then watch only key. To watch keys with a common prefix, useget_start_and_end_of_prefix to generate [key, key_end).
• filter_type: filter the type of the watching events, which is one of the following:
  • ALL: receives all kinds of update events.
  • UPDATE: filter only updates event.
  • DELETE: filter only deletes event.

Every time a watch request is received, a Grpc stream is created. Every time the watched key changes, databend-meta uses the created stream to notify the client.