Sync SSL certificates on events

[antoineco]

What this PR does / why we need it:

The background check for missing secrets leads to unnecessarily delayed synchronization of SSL secrets.

Illustrated scenario:

1. An Ingress object with a reference to a Secret is created, the Secret does not exist yet
2. A config synchronization is triggered, all sections supposed to use the Secret are ignored
3. The Secret is created, but nothing happens until the next background check

This PR improves the tracking of Ingress->Secret references using a new ObjectRefMap interface. At all times the map contains an up-to-date list of Secrets referenced by one or more Ingresses, including Secrets that do not exist within the cluster.

All EventHandlers for Secret resources perform a lookup in the map in order to determine whether the object that triggered the event is being referenced, in which case:

• annotations for matching Ingresses are extracted again
• secrets for matching Ingresses are synced again (possibly leading to new a config synchronization in case the content of the Secret differs)

The result is a more immediate feedback to changes that does not rely on arbitrary delays.

[antoineco]

Failing some unrelated test, rebasing on master.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@8855460). Click here to learn what that means.
The diff coverage is 65%.

@@           Coverage Diff            @@
##             master   #2342   +/-   ##
========================================
  Coverage          ?   39.1%           
========================================
  Files             ?      73           
  Lines             ?    5202           
  Branches          ?       0           
========================================
  Hits              ?    2034           
  Misses            ?    2878           
  Partials          ?     290

Impacted Files	Coverage Δ
internal/task/queue.go	79.41% <ø> (ø)
internal/ingress/sslcert.go	100% <ø> (ø)
internal/ingress/controller/controller.go	0% <0%> (ø)
internal/ingress/controller/store/backend_ssl.go	42.04% <33.33%> (ø)
internal/ingress/controller/store/store.go	51.62% <56.34%> (ø)
internal/ingress/controller/store/objectref.go	93.75% <93.75%> (ø)

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[aledbf]

/approve

[aledbf]

/lgtm

[aledbf]

@antoineco thanks!

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aledbf, antoineco

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