replace std RwLock and Mutex with parking-lot

[parmesant]

Debugging the too many open files (os error 24) error which we got due to a network issue
The machine was running ingest server along with fluent-bit

Fixes #XXXX.

Description

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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.

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