Allow transaction for offchain indexing (#7290)

* Moving offchain change set to state machine overlay change set,
preparing use of change set internally.

* Make change set generic over key and value, and use it for offchain
indexing.

* test ui change

* remaining delta

* generating with standard method

* Remove 'drain_committed' function, and documentation.

* Default constructor for enabling offchain indexing.

* Remove offchain change specific iterators.

* remove pub accessor

* keep previous hierarchy, just expose iterator instead.

* Update primitives/state-machine/src/overlayed_changes/mod.rs

Co-authored-by: Tomasz Drwięga <tomusdrw@users.noreply.github.com>

* fix line break

* missing renamings

* fix import

* fix new state-machine tests.

* Don't expose InnerValue type.

* Add test similar to set_storage.

* Remove conditional offchain storage (hard to instantiate correctly).

* fix

* offchain as children cannot fail if top doesn't

Co-authored-by: Addie Wagenknecht <addie@nortd.com>
Co-authored-by: Tomasz Drwięga <tomusdrw@users.noreply.github.com>

substrate/primitives/offchain/src/lib.rs

@@ -21,7 +21,7 @@
 #![warn(missing_docs)]
 
 /// Re-export of parent module scope storage prefix.
-pub use sp_core::offchain::STORAGE_PREFIX as STORAGE_PREFIX;
+pub use sp_core::offchain::STORAGE_PREFIX;
 
 sp_api::decl_runtime_apis! {
 	/// The offchain worker api.