kvserver: zero out slice before reuse

Standard Go error - we were trying to avoid allocations by recycling a
slice but weren't zeroing it out before. The occasional long slice that
would reference a ton of memory would then effectively keep that large
amount of memory alive forever.

Touches #71050.

Release note: None

pkg/kv/kvserver/raft_transport.go

@@ -504,12 +504,18 @@ func (t *RaftTransport) processQueue(
 			}
 
 			err := stream.Send(batch)
-			batch.Requests = batch.Requests[:0]
-
-			atomic.AddInt64(&stats.clientSent, 1)
 			if err != nil {
 				return err
 			}
+
+			// Reuse the Requests slice, but zero out the contents to avoid delaying
+			// GC of memory referenced from within.
+			for i := range batch.Requests {
+				batch.Requests[i] = RaftMessageRequest{}
+			}
+			batch.Requests = batch.Requests[:0]
+
+			atomic.AddInt64(&stats.clientSent, 1)
 		}
 	}
 }