AbstractRenditionBuilderWork must use unique work id

This class is used to build rendition worker and create a work id based on document id and a variant id (like user id).
The changetoken should be used to not generate identical id for different rendition.
(StreamWorkManager will skip duplicates work ids)

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In the case of Rendition we want to execute multiple times the exact same work because the results is in transient store.
To handle this, the Work has a new flag isIdempotent which is true by default. So StreamWorkManager will skip duplicate only for idempotent work.
And AbstractRenditionBuilderWork has the flag set to false.