fix copy paste error in v3/v4 and benchmark duplicate merges

Merging 1000 duplicates of the same batch:
v1: 98ms
v2: 101ms
v3: 116ms
v4: 116ms

So as expected v3 and v4 have a small cost with duplicates for optimizing for non-overlapping,
but even with ridiculous numbers duplicates are fast to process anyway.

merge_v3.py

@@ -57,13 +57,13 @@ def merge_messages(*batches):
 				if id is None:
 					merged = merge_message(message, candidate)
 					if merged is not None:
-						candidates.pop(index)
+						batch.pop(index)
 						break
 				elif (candidate.get("tags") or {}).get("id") == id:
 					merged = merge_message(message, candidate)
 					if merged is None:
 						raise ValueError("TODO")
-					candidates.pop(index)
+					batch.pop(index)
 					break
 			if merged is not None:
 				message = merged

merge_v4.py

@@ -85,13 +85,13 @@ def merge_messages(*batches):
 				if id is None:
 					merged = merge_message(message, candidate)
 					if merged is not None:
-						candidates.pop(index)
+						batch.pop(index)
 						break
 				elif (candidate.get("tags") or {}).get("id") == id:
 					merged = merge_message(message, candidate)
 					if merged is None:
 						raise ValueError("TODO")
-					candidates.pop(index)
+					batch.pop(index)
 					break
 			if merged is not None:
 				message = merged