1. Remove proof extension as it is not needed. Host are required to provide proofs from the moment they fill a slot, for the duration specified by the contract. This means that the ending of their requirements will be staggered at the end, as they were at the start, but this is more predicable for determining the cost of a request.
2. The proof end time was modified so that if the request state is not accepting proofs, it takes the min of the slot proof end time, the request end time, or block.timestamp - 1, which ensures that it returns a time in the past. If the slot is accepting proofs, it returns the slot end time.
3. Modify marketplace tests so that `waitUntilFinished` advances time to the proof ending of the last slot filled.
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