Pov recovery for parachains (#445)

* Start with a failing integration test & some refactorings

* More work

* Make it "work"

* Add `NullConsensus` for the test

* More refactorings

* Move stuff over to its own crate

* Refactorings

* Integrate it into `service` and make the test working

* Docs and some exit condition

* Use the real import queue

* Fix tests

* Update client/pov-recovery/src/active_candidate_recovery.rs

Co-authored-by: Bernhard Schuster <bernhard@ahoi.io>

* Fetch slot duration from the relay chain

* Docs

* Fixes

Co-authored-by: Bernhard Schuster <bernhard@ahoi.io>

primitives/core/src/lib.rs

@@ -215,6 +215,11 @@ impl<B: BlockT> ParachainBlockData<B> {
 		}
 	}
 
+	/// Convert `self` into the stored block.
+	pub fn into_block(self) -> B {
+		B::new(self.header, self.extrinsics)
+	}
+
 	/// Convert `self` into the stored header.
 	pub fn into_header(self) -> B::Header {
 		self.header