Expose that BasicQueue expects blocking spawn (#5860)

* Expose that `BasicQueue` expects blocking spawn

Up to now `BasicQueue` expected a closure that to spawn a `Future`.
This was expected to be a closure that spawns a blocking future.
However, this wasn't documented anywhere. This pr introduces a new trait
`SpawnBlocking` that exposes this requirement to the outside.

* Feedback

substrate/client/consensus/babe/src/lib.rs

@@ -107,7 +107,7 @@ use sc_client_api::{
 };
 use sp_block_builder::BlockBuilder as BlockBuilderApi;
 
-use futures::{prelude::*, future::BoxFuture};
+use futures::prelude::*;
 use log::{debug, info, log, trace, warn};
 use sc_consensus_slots::{
 	SlotWorker, SlotInfo, SlotCompatible, StorageChanges, CheckedHeader, check_equivocation,
@@ -1272,7 +1272,7 @@ pub fn import_queue<Block: BlockT, Client, Inner>(
 	finality_proof_import: Option<BoxFinalityProofImport<Block>>,
 	client: Arc<Client>,
 	inherent_data_providers: InherentDataProviders,
-	spawner: impl Fn(BoxFuture<'static, ()>) -> (),
+	spawner: &impl sp_core::traits::SpawnBlocking,
 ) -> ClientResult<BabeImportQueue<Block, sp_api::TransactionFor<Client, Block>>> where
 	Inner: BlockImport<Block, Error = ConsensusError, Transaction = sp_api::TransactionFor<Client, Block>>
 		+ Send + Sync + 'static,