Add a pool idle timeout for subgraph HTTP connectors

[garypen]

Having a high idle pool timeout duration can sometimes trigger situations in which an HTTP request cannot complete (see this comment for more information).

This changeset sets a default timeout duration of 5 seconds, which we may make configurable eventually.

fixes: #3435

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

Note any exceptions here

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
• 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