Allow proof ending to be extending once a contract is started, so that all filled slots share an ending time that is equal to the contract end time. Added tests.
Add a mapping for proof id to endId so that proof expectations can be extended for all proofs that share a given endId.
Add function modifiers that require the request state to allow proofs, with accompanying tests.
General clean up of each function’s request state context, with accompanying tests.
General clean up of all tests, including state change “wait” functions and normalising the time advancement functions.
Add function `isFinished` that checks whethere the state has been set to `RequestState.Finished` (which is not being set yet) or if the contract was started (`RequestState.Started`) and the contract duration has lapsed.
To facilitate `isFinished`, the contract start time (`startedAt`) was added to calculate the contract end time.
The block hash of the current block is not known yet, so
we use the block hash of the previous block to determine
whether a proof is required for a block.
