[FLINK-14504][rest] Add Cluster DataSet REST API

[zentol]

Adds the REST API for managing cluster partitions. The collection of all cluster partitions belonging to a single data set is referred to as a "Cluster DataSet".

The API allows the user to list and delete partitions. The handlers are relatively straight-forward and pretty much just forward calls to the RM (which forwards them to the Tracker), at most converting some data-structures.

As a prerequisite we no longer filter incomplete data sets in PartitionTracker#listDataSets.
We don't have a mechanism to ensure that either all partitions of a data set are registered or the remaining partitions are released at some point.
In these cases the user has to step in, but without being able to list incomplete partitions there's nothing that can be done.
To differentiate between complete/incomplete partitions we expose the number of currently registered partitions.
This count is only determined when the listing is requested for performance and complexity reasons.

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Last check on commit 44066c5 (Fri Apr 03 10:45:56 UTC 2020)

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