Fix race condition in timeoutHandler.ServeHTTP

[JRBANCEL]

Proposed Changes

• If a panic happens in the inner handler after a timeout, panicChan will be closed because ServeHTTP would have returned and called defer close(panicChan). Then, the write to the closed panicChan will panic. A channel should always be closed in the sending goroutine if there is a single sender.
• Simplified the code by using a single channel

/assign @markusthoemmes
/assign @vagababov

[knative-prow-robot]

@JRBANCEL: 0 warnings.

In response to this:

Proposed Changes

• If a panic happens in the inner handler after a timeout, panicChan will be closed because ServeHTTP would have returned and called defer close(panicChan). Then, the write to the closed panicChan will panic. A channel should always be closed in the sending goroutine if there is a single sender.
• Simplified the code by using a single channel

/assign @markusthoemmes
/assign @vagababov

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.

[knative-test-reporter-robot]

The following jobs failed:

Test name	Triggers	Retries
pull-knative-serving-unit-tests		0/3

Failed non-flaky tests preventing automatic retry of pull-knative-serving-unit-tests:

pkg/reconciler/autoscaling/kpa.TestControllerSynchronizesCreatesAndDeletes

[knative-metrics-robot]

The following is the coverage report on the affected files.
Say /test pull-knative-serving-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/queue/timeout.go	90.9%	90.7%	-0.2

[vagababov]

/lgtm

[vagababov]

/approve

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: JRBANCEL, vagababov

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:

• pkg/queue/OWNERS [vagababov]

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