Allow unlimited connections per-worker

[sjmonson]

Summary

By default each httpx client supports a maximum of 100 connections (ref). We want this uncapped as connection count is maintained by a semaphore.

Test Plan

• See Per-worker concurrency is capped at 100 #487

Related Issues

• Resolves Per-worker concurrency is capped at 100 #487

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[jaredoconnell]

This change will reduce the keepalive from the default of 5 seconds to a half of a second. Do you anticipate any side effects from the shortened keepalive?

[sjmonson]

This change will reduce the keepalive from the default of 5 seconds to a half of a second. Do you anticipate any side effects from the shortened keepalive?

I don't know. Need to come up with some manual tests to verify that this is ok. Since max_keepalive is set to None, the intention is that either the connection gets reused immanently or we expire it to avoid holding a large number of idle connections. max_keepalive is None because the default is 20 and I feel that is too low for our use-case. Setting it to any other number seems arbitrary.