[FLINK-18533][coordination] Tolerate pending deployments being reported

[zentol]

Fixes an issue where a task would be canceled if a task executor reported the execution via heartbeats without the acknowledgement yet being processed by the JobMaster. This can happen because the Acknowledge was lost, or due to message re-ordering on the JobMaster side.

The ExecutionDeployment(Tracker/Reconciler) now distinguish between PENDING/DEPLOYED executions.
For reconciliation purposes executions in a PENDING state are ignored.
Executions are moved into a deployed state once the acknowledge by the TaskExecutor has been processed by the JobMaster.
If the task is never acknowledged, then, as before, the task is failed by the JobMaster, and then removed from the tracker (due to terminal state transition, as before).

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Last check on commit f681caa (Tue Jul 21 12:03:44 UTC 2020)

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[tillrohrmann]

Thanks for creating this PR @zentol. The changes look good to me. +1 for merging.