Use common prefixes for logstream listing

[trueleo]

Fixes #51.

Description

Fixes logstream listing by using common prefixes.

This lists all the root level objects which in case of parseable are logstream directories.

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This PR has:

• been tested to ensure log ingestion and log query works.
• added comments explaining the "why" and the intent of the code wherever would not be obvious for an unfamiliar reader.
• added documentation for new or modified features or behaviors.

[nitisht]

Great find!