Diagnostics quality of life improvements (#2375)

* Implement `Debug` manually for CandidateHash

This will make candidate hashes printed consistently without the
`CandidateHash(` and `)` decorations.

* Do not print CompressedPov's guts

It can be overwhelming. Better just use the size.

* Log when candidate is generated

* Print para_id and candidate_hash upon receiving a collation

polkadot/node/network/protocol/src/lib.rs

@@ -327,7 +327,7 @@ pub mod v1 {
 	}
 
 	/// SCALE and Zstd encoded [`PoV`].
-	#[derive(Debug, Clone, Encode, Decode, PartialEq, Eq)]
+	#[derive(Clone, Encode, Decode, PartialEq, Eq)]
 	pub struct CompressedPoV(Vec<u8>);
 
 	impl CompressedPoV {
@@ -374,6 +374,12 @@ pub mod v1 {
 		}
 	}
 
+	impl std::fmt::Debug for CompressedPoV {
+    	fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+			write!(f, "CompressedPoV({} bytes)", self.0.len())
+		}
+	}
+
 	/// Network messages used by the collator protocol subsystem
 	#[derive(Debug, Clone, Encode, Decode, PartialEq, Eq)]
 	pub enum CollatorProtocolMessage {