⚠️ Propagate context on Manager.Start(...)

[vincepri]

This change reshuffles how the manager accepts and operates on
a context. With this change, the user experience is greatly improved,
users can now use ctrl.SetupSignalHandler() to create a context, enrich
it if they want to, and pass it to manager.Start.

In addition, this PR changes how the context and stop channel are
handled internally to ensure proper cancellation.

/assign @alvaroaleman
/milestone v0.7.x

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: vincepri

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [vincepri]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[k8s-ci-robot]

@vincepri: The following test failed, say /retest to rerun all failed tests:

Test name	Commit	Details	Rerun command
pull-controller-runtime-apidiff-master	0f46012	link	/test pull-controller-runtime-apidiff-master

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository. I understand the commands that are listed here.

[alvaroaleman]

/lgtm

[EronWright]

Would like to point out that this PR may have a secondary positive effect on test suites which utilize the controller test framework. By propagating the manager's cancelable context to the event sources, and then to the cache during its initial sync, we avoid a possible deadlock due to the cache sync occurring after the manager (and its cache) has been stopped. In other words, folks should see fewer spurious test failures.

[vincepri]

@EronWright Thanks for pointing that out! I actually noticed the same on some flaky test within the Cluster API project, the other PR that merged a few days ago #1354 also helps if you need to wait for the cache to sync during tests.