Add --idle-request-interval-secs

[XAMPPRocky]

This PR adds a new flag to both relay and load balancer services.

--idle-request-interval-secs adds a timeout for waiting for responses, so that if the service hasn't heard back from a management server in the specified time it will send a request for new resources. This should help catch any spurious errors where a service is unresponsive but the HTTP connection is active, since the request failing will trigger a new HTTP connection.

[quilkin-bot]

Build Succeeded 🥳

Build Id: 4cc3e5e5-237c-4e07-b02d-e330f1d789e1

The following development images have been built, and will exist for the next 30 days:

• us-docker.pkg.dev/quilkin/ci/quilkin:0.7.0-dev-67ea8c7

To build this version:

git fetch git@github.com:googleforgames/quilkin.git pull/790/head:pr_790 && git checkout pr_790
cargo build