Add greedy mempool with configuration options

[trueleo]

Description

Adds a usable memory limit to each query. If P_QUERY_MEMORY_LIMIT is set then it will use that as a fixed limit for greedy memory pool otherwise it will use 80% of available memory as hard limit for this query. In effect if the memory pool quota is filled it then datafusion will try to spill few execution nodes in temporary directory.

Note -

This solution is not dynamic as Datafusion does not keep track of actual available memory during its runtime. Pool size set to Datafusion is a fixed number in bytes.

So it can happen that bound was set higher but some other process took memory and thus actual allocation cannot happen at runtime, Datafusion will return an error in that case instead of OOM.
Vice versa if bound was set lower and later during runtime if more memory is available for use even then it won't be able to use it

Alternative:

We can keep this option to set limit but when it is not set then we just use the default Unbounded memory manager

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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.