Require the main (GraphQL) server to shutdown before other servers

[garypen]

We'd like to have more control over the sequencing of server shutdown so that the main (GraphQL) server is required to complete before other servers are shutdown.

This is particularly desirable when the router is executing in Kubernetes, since continuing to report live/ready checks during shutdown is a requirement.

fixes: #3521

Checklist

Complete the checklist (and note appropriate exceptions) before a final PR is raised.

• Changes are compatible[^1]
• Documentation[^2] completed
• Performance impact assessed and acceptable
• Tests added and passing[^3]
  • Unit Tests
  • Integration Tests
  • Manual Tests

Exceptions

Difficult to assess the impact with automated testing. All existing tests pass and I performed manual testing as follows:

1. Run a router

cargo run -- -s examples/graphql/local.graphql -c router.yaml --hot-reload

Note: Config doesn't matter, I'm just passing one to have a touch target. --hot-reload is to enable reloading

2. Exercise the health with this script:

while true; do curl "http://localhost:8088/health?ready" || {echo "curl failed"; break}; done

3. In a separate terminal force reloads by touching the config file.

Observe that the curl loop keeps working and doesn't exit through multiple reloads of the router before the router is actually shutdown.

Notes

[^1]. It may be appropriate to bring upcoming changes to the attention of other (impacted) groups. Please endeavour to do this before seeking PR approval. The mechanism for doing this will vary considerably, so use your judgement as to how and when to do this.
[^2]. Configuration is an important part of many changes. Where applicable please try to document configuration examples.
[^3]. Tick whichever testing boxes are applicable. If you are adding Manual Tests:
- please document the manual testing (extensively) in the Exceptions.
- please raise a separate issue to automate the test and label it (or ask for it to be labeled) as manual test

[router-perf]

CI performance tests

• xxlarge-request - Stress test with 100 MB request payload
• step - Basic stress test that steps up the number of users over time
• events_without_dedup - Stress test for events with a lot of users and deduplication DISABLED
• events - Stress test for events with a lot of users and deduplication ENABLED
• large-request - Stress test with a 1 MB request payload
• const - Basic stress test that runs with a constant number of users
• no-graphos - Basic stress test, no GraphOS.
• 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
• xlarge-request - Stress test with 10 MB request payload
• 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

[o0Ignition0o]

Super nice 🚀