Add threadpool to `POST /batch/queue`

[jonathan-d-zhang]

Send requests concurrently rather than synchronously. This provides a O(n) speedup.

Closes #208

[sentry-io[bot]]

Suspect Issues

This pull request was deployed and Sentry observed the following issues:

• ‼️ ValueError: max_workers must be greater than 0 /batch/package View Issue

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