Prepare syncing for parallel sync strategies (#3224)

This PR should supersede
https://github.com/paritytech/polkadot-sdk/pull/2814 and accomplish the
same with less changes. It's needed to run sync strategies in parallel,
like running `ChainSync` and `GapSync` as independent strategies, and
running `ChainSync` and Sync 2.0 alongside each other.

The difference with https://github.com/paritytech/polkadot-sdk/pull/2814
is that we allow simultaneous requests to remote peers initiated by
different strategies, as this is not tracked on the remote node in any
way. Therefore, `PeerPool` is not needed.

CC @skunert

---------

Co-authored-by: Sebastian Kunert <skunert49@gmail.com>

substrate/client/network/sync/src/strategy/state.rs

@@ -330,11 +330,6 @@ impl<B: BlockT> StateStrategy<B> {
 		}
 	}
 
-	/// Get the number of peers known to syncing.
-	pub fn num_peers(&self) -> usize {
-		self.peers.len()
-	}
-
 	/// Get actions that should be performed by the owner on [`WarpSync`]'s behalf
 	#[must_use]
 	pub fn actions(&mut self) -> impl Iterator<Item = StateStrategyAction<B>> {