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/core-primitives/src/lib.rs

@@ -60,7 +60,7 @@ pub type Hash = sp_core::H256;
 /// This type is produced by [`CandidateReceipt::hash`].
 ///
 /// This type makes it easy to enforce that a hash is a candidate hash on the type level.
-#[derive(Clone, Copy, Encode, Decode, Hash, Eq, PartialEq, Debug, Default)]
+#[derive(Clone, Copy, Encode, Decode, Hash, Eq, PartialEq, Default)]
 #[cfg_attr(feature = "std", derive(MallocSizeOf))]
 pub struct CandidateHash(pub Hash);
 
@@ -71,6 +71,12 @@ impl std::fmt::Display for CandidateHash {
 	}
 }
 
+impl sp_std::fmt::Debug for CandidateHash {
+    fn fmt(&self, f: &mut sp_std::fmt::Formatter<'_>) -> sp_std::fmt::Result {
+        write!(f, "{:?}", self.0)
+    }
+}
+
 /// Index of a transaction in the relay chain. 32-bit should be plenty.
 pub type Nonce = u32;