Query-time clears (latest-at only)

[teh-cmc]

Resolves pending clears at query time, rather than through write cascades.

Only works for latest-at queries for now. We'll take care of range queries once the deserialization cache has been removed, it's just not worth dealing with all the extra complexity at the moment.
The only visible change for users is that the "scalar plot hole" feature doesn't work anymore.

• Fixes Clear logic needs to be handled by the store and/or query engine #6454
• Fixes Decide what should show up on time widget, and when #4220
• Fixes Specify the expected semantics of clear APIs in tests #3287

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[jleibs]

This is surprisingly straight forward at this point relative to the pending-clear complexity we've been carrying for the last 18 months.