Parse now and duration in start and end time

[trueleo]

Fixes #490 .

Description

Allow user to specify now and duration in endTime and startTime fields instead of giving concrete timestamps.

Note that this only affects the timerange for internal file lookups and does not guarantee restriction exact timeframe for p_timestamp for the query. In that case user is expected to pass in where p_timestamp < now() and p_timestamp > (now() - interval '10 minute')

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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.