Create all temporary directories on startup

[trueleo]

Fixes #74.

Description

Create all the temporary directories on program startup. Right now event writer simply creates the record file without checking or creating the directories. This responsibility should be delegated to startup sync where it creates all the directories beforehand based on metadata available.

Startup sync now explicitly panics if create temporary directory fails ( permission issues ) or moving some old record file fails. This is because these operations are essential for running the server and it saves users from the confusion of why they cannot post any event.

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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]

LGTM