Multi-phase elections solution resubmission (#8290)

* not climate

* explain the intent of the bool in the unsigned phase

* remove glob imports from unsigned.rs

* add OffchainRepeat parameter to ElectionProviderMultiPhase

* migrate core logic from #7976

This is a much smaller diff than that PR contained, but I think
it contains all the essentials.

* improve formatting

* fix test build failures

* cause test to pass

* Apply suggestions from code review

Co-authored-by: Kian Paimani <[email protected]>

* collapse imports

* threshold acquired directly within try_acquire_offchain_lock

* add test of resubmission after interval

* add test that ocw can regenerate a failed cache when resubmitting

* ensure that OCW solutions are of the correct round

This should help prevent stale cached solutions from persisting
past the election for which they are intended.

* add test of pre-dispatch round check

* use `RawSolution.round` instead of redundantly externally

* unpack imports

Co-authored-by: Kian Paimani <[email protected]>

* rename `OFFCHAIN_HEAD_DB` -> `OFFCHAIN_LOCK`

* rename `mine_call` -> `mine_checked_call`

* eliminate extraneous comma

* check cached call is current before submitting

* remove unused consts introduced by bad merge.

Co-authored-by: Guillaume Thiolliere <[email protected]>

* resubmit when our solution beats queued solution

* clear call cache if solution fails to submit

* use local storage; clear on ElectionFinalized

* Revert "use local storage; clear on ElectionFinalized"

This reverts commit 4b46a9388532d0c09b337dc7c7edf76044a6cee8.

* BROKEN: try to filter local events in OCW

* use local storage; simplify score fetching

* fix event filter

* mutate storage instead of setting it

* StorageValueRef::local isn't actually implemented yet

* add logging for some events of interest in OCW miner

* rename kill_solution -> kill_ocw_solution to avoid ambiguity

* defensive err instead of unreachable given unreachable code

* doc punctuation

Co-authored-by: Kian Paimani <[email protected]>

* distinguish miner errors between "out of date" and "call invalid"

* downgrade info logs -> debug

* ensure encoded call decodes as a call

* fix semantics of validation of pre-dispatch failure for wrong round

* move score check within `and_then`

* add test that offchain workers clear their cache after election

* ensure that bad ocw submissions are not retained for resubmission

* simplify fn ocw_solution_exists

* add feasibility check when restoring cached solution

should address https://github.com/paritytech/substrate/pull/8290/files#r617533358



restructures how the checks are sequenced, which simplifies legibility.

* simplify checks again

Co-authored-by: Kian Paimani <[email protected]>
Co-authored-by: Guillaume Thiolliere <[email protected]>