Persistent Local Storage for offchain workers. (#2894)

* WiP.

* Implement offchain storage APIs.

* Change compare_and_set to return bool.

* Add offchain http test.

* Fix tests.

* Bump spec version.

* Fix warnings and test.

* Fix compilation.

* Remove unused code.

* Introduce Local (fork-aware) and Persistent storage.

* Fix borked merge.

* Prevent warning on depreacated client.backend

* Fix long lines.

* Clean up dependencies.

* Update core/primitives/src/offchain.rs

Co-Authored-By: André Silva <[email protected]>

* Update core/primitives/src/offchain.rs

Co-Authored-By: André Silva <[email protected]>