Paused states subclass Scheduled

[jlowin]

Paused states can have fixed expirations

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What does this PR change?

Closes #1342 and allows pauses to have pre-defined expirations.

[cicdw]

Two requests:

• I believe you'll need to update the Paused state serialization schema as well.
• could you add an optional start_time kwarg to the pause_task utility located in utilities/tasks.py?

Otherwise, it's awesome how simple this is!

[codecov]

Codecov Report

Merging #1375 into master will decrease coverage by 0.01%.
The diff coverage is 94.44%.