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/collator-protocol/src/validator_side.rs

@@ -387,6 +387,9 @@ where
 					tracing::debug!(
 						target: LOG_TARGET,
 						%request_id,
+						?para_id,
+						?relay_parent,
+						candidate_hash = ?receipt.hash(),
 						"Received collation",
 					);