Add BFS fallback for getting node downstreams with high fanout

[shangyian]

Summary

When retrieving downstreams for a node, nodes with large downstream graphs will cause the recursive CTE query to hang and eventually fail, after a significant period of database processing.

This PR introduces a BFS-based fallback for retrieving downstream nodes in cases where the node's initial set of children exceeds a configurable fanout threshold. This is initial count is being used as a proxy for the final size of the node's downstream graph.

A BFS approach allows processing level by level, reducing load on the database in cases of excessive recursion. The nodes on each level are processed concurrently, with max concurrency configurable at the server level.

Test Plan

Added a test to compare results from the BFS vs recursive CTE approaches

• PR has an associated issue: #
• make check passes
• make test shows 100% unit test coverage

Deployment Plan

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