Fix scheduling of reconciliation

[sheerun]

This is improved version of #1543. It makes sure that:

1. At most one reconciliation is executed at once (RunOnce is called in single place)
2. Multiple reconciliation are batched (for now batching window is hardcored to 5s)
3. We move the responsibility of scheduling, batching and throttling reconciliations from Source to Controller by moving it from ingress.go and service.go to controller.go and changing interface of AddEventHandler(handler func() error, stopChan <-chan struct{}, minInterval time.Duration) to AddEventHandler(ctx context.Context, handler func())
4. We switch from stopChan to cancellable context
5. bounded_frequency_runner.go is no longer necessary, all throttling logic is inside Controller
6. We move TestSourceEventHandler to source_test.go (where it belongs), and add test for the behavior above

#1568 Should be merged first

[sheerun]

/assign @Raffo

[sheerun]

I think this is ready to be reviewed and merged

[sheerun]

@tariq1890 Please refresh, I've rebased this on master

[tariq1890]

Changes LGTM. deferring to @njuettner or @Raffo

[sheerun]

@tariq1890 @ytsarev For the future: you can use /lgtm to inform bot as well :)

[Raffo]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Raffo, sheerun

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 [Raffo]

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