Use new bookkeeper fluent API

[sijie]

FEATURE REQUEST

1. Please describe the feature you are requesting.

A new bookkeeper fluent API is introduced in apache/bookkeeper#506. DistributedLog should integrate with the new fluent API to leverage CompletableFuture.

This task can also be used as a sanity check for verifying if the new bookkeeper API can take replace of the existing API.

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?

This should be a blocker for release/0.6.0.

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

N/A