client: implement API to get the front and back of logs in a topic

[ijsong]

Sometimes we want to know the front and back of logs in a topic without fetching anything. For example, clients can subscribe to logs starting from either the first log entry sequence number or the last log entry sequence number.

Possible limitations of this API

• Stale information: Since the end of logs is growing continuously, the back can be less than the recent one. The Trim API can also remove the front of logs.
• Temporarily unavailable: This API cannot determine the front and back of the topic even if at least one of the log streams in a topic cannot return its low watermark and high watermark correctly. This situation hardly occurs since, to do that, all replicas in a log stream either should not be accessed or should not know their last committed log.