Make it possible in the API to query for tasks by child_id (aka object id)
Make it possible in the API to query for tasks by child_id (aka object id)

Summary:
This makes it much easier to build external tooling to inspect Task state.
Prior to this, it could require 10s of thousands of requests to find the Task
you're looking for.

Test Plan: Used it locally.

Reviewers: nate

Reviewed By: nate

Subscribers: changesbot

Differential Revision: https://tails.corp.dropbox.com/D161628

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