Support open a log stream in overwrite mode

[sijie]

FEATURE REQUEST

1. Please describe the feature you are requesting.

in supporting #227 , opening a log stream with overwrite mode is done with two separate operations - delete the log stream first and recreate one. there is no atomicity on those two operations. It would be great if dlog api can support opening a log stream with overwrite.

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.