Make Proposer consume its reference on propose (#6190)

* Make `Proposer` consume its reference on `propose`

A proposer must be created per new round, so it makes sense to have the
proposer consume its own reference.

* Remove `ProposerInner`

substrate/primitives/consensus/common/src/lib.rs

@@ -157,7 +157,7 @@ pub trait Proposer<B: BlockT> {
 	///
 	/// Returns a future that resolves to a [`Proposal`] or to [`Error`].
 	fn propose(
-		&mut self,
+		self,
 		inherent_data: InherentData,
 		inherent_digests: DigestFor<B>,
 		max_duration: Duration,