Ensure election offchain workers don't overlap (#8828)

* Initial version, well tested, should work fine.

* Add one last log line

* Update frame/election-provider-multi-phase/src/unsigned.rs

Co-authored-by: Gavin Wood <gavin@parity.io>

* Update frame/election-provider-multi-phase/src/unsigned.rs

Co-authored-by: Guillaume Thiolliere <gui.thiolliere@gmail.com>

* Update frame/election-provider-multi-phase/src/unsigned.rs

Co-authored-by: Guillaume Thiolliere <gui.thiolliere@gmail.com>

* Fix a few more things

* fix build

* rewrite the whole thing with a proper lock

* clean

* clean some nits

* Add unit tests.

* Update primitives/runtime/src/offchain/storage_lock.rs

Co-authored-by: Peter Goodspeed-Niklaus <coriolinus@users.noreply.github.com>

* Apply suggestions from code review

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

* Fix test

* Fix tests

Co-authored-by: Gavin Wood <gavin@parity.io>
Co-authored-by: Guillaume Thiolliere <gui.thiolliere@gmail.com>
Co-authored-by: Peter Goodspeed-Niklaus <coriolinus@users.noreply.github.com>
Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>