Reintroduce "use spawn_blocking for parsing"

[abernix]

Important

This is ready to review once #5638 lands.

Re-lands #5582 after it was reverted in #5643 as it seems to have been causing tests to flake at a somewhat pronounced rate on dev — always on Linux. We believe it's just actually the tests, or the way we tested, but we'll revert this for now so dev isn't flaking for everyone trying to develop against it.

And worth mentioning again, this was was originally @xuorig's in #5235 which @Geal took over and @garypen ultimately landed in #5582 .

Marking this as a draft for someone to look at it.

[router-perf]

CI performance tests

• const - Basic stress test that runs with a constant number of users
• demand-control-instrumented - A copy of the step test, but with demand control monitoring and metrics enabled
• demand-control-uninstrumented - A copy of the step test, but with demand control monitoring enabled
• enhanced-signature - Enhanced signature enabled
• events - Stress test for events with a lot of users and deduplication ENABLED
• events_big_cap_high_rate - Stress test for events with a lot of users, deduplication enabled and high rate event with a big queue capacity
• events_big_cap_high_rate_callback - Stress test for events with a lot of users, deduplication enabled and high rate event with a big queue capacity using callback mode
• events_callback - Stress test for events with a lot of users and deduplication ENABLED in callback mode
• events_without_dedup - Stress test for events with a lot of users and deduplication DISABLED
• events_without_dedup_callback - Stress test for events with a lot of users and deduplication DISABLED using callback mode
• extended-reference-mode - Extended reference mode enabled
• large-request - Stress test with a 1 MB request payload
• no-tracing - Basic stress test, no tracing
• reload - Reload test over a long period of time at a constant rate of users
• step-jemalloc-tuning - Clone of the basic stress test for jemalloc tuning
• step-local-metrics - Field stats that are generated from the router rather than FTV1
• step-with-prometheus - A copy of the step test with the Prometheus metrics exporter enabled
• step - Basic stress test that steps up the number of users over time
• xlarge-request - Stress test with 10 MB request payload
• xxlarge-request - Stress test with 100 MB request payload

[garypen]

The code is fine. It was a source of testing fails because it triggered the behaviour described in #5638. As soon as #5638 merges, we should merge this.

[garypen]

#5638 must merge before this.