Support truncate a log stream at the tail

[sijie]

FEATURE REQUEST

1. Please describe the feature you are requesting.

dlog api supports truncation at the head. it is natural for a streaming use case to purge old data. it would be good to support truncation at the tail as well, like what filesystem truncate operation.

2. Indicate the importance of this issue to you (blocker, must-have, should-have, nice-to-have). Are you currently using any workarounds to address this issue?

nice-to-have

3. Provide any additional detail on your proposed use case for this feature.

it is a feature for #227