[FLINK-19536][network] Fix inconsistent priority notification.

[AHeise]

What is the purpose of the change

This commit fixes a range of smaller bugs introduced in #13228. They all affect the (priority) availability of InputGate and trigger various kinds of preconditions that have been in embedded into the network stack.

Brief change log

• Fix union gate polling gates through iterator. That may leave PriorityDeque in inconsistent state.
• Double-check if gate still has priority buffer when enqueuing in UnionInputGate. Since notification is not atomic in respect to gate enqueuing, priority event already polled by task thread when netty enqueues the gate.
• Only notify data available when there is a non-priority buffer. This avoids StreamTaskNetworkInput to poll empty gates.

Verifying this change

• Added tests for each bug (WIP).

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): (yes / no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (yes / no)
• The serializers: (yes / no / don't know)
• The runtime per-record code paths (performance sensitive): (yes / no / don't know)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn/Mesos, ZooKeeper: (yes / no / don't know)
• The S3 file system connector: (yes / no / don't know)

Documentation

• Does this pull request introduce a new feature? (yes / no)
• If yes, how is the feature documented? (not applicable / docs / JavaDocs / not documented)

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Last check on commit eef43a8 (Thu Oct 08 13:14:19 UTC 2020)

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[AHeise]

Subsumed by #14073 and #14141.