rangefeed: omit timestamps on previous values

There was a discrepancy with whether or not the previous contained the
timestamp of when the previous value was recorded. With the initial
scan, we populated the timestamp of the initial scan itself. With
regular diffs, we omitted the timestamp. This made for confusing
semantics (well, confusing at least for this author).

Release note: None

pkg/kv/kvclient/rangefeed/rangefeed.go

@@ -275,6 +275,7 @@ func (f *RangeFeed) maybeRunInitialScan(
 		// indicating that the value was previously deleted.
 		if f.withDiff {
 			v.PrevValue = v.Value
+			v.PrevValue.Timestamp = hlc.Timestamp{}
 		}
 
 		// It's something of a bummer that we must allocate a new value for each

pkg/kv/kvclient/rangefeed/rangefeed_external_test.go

@@ -88,8 +88,9 @@ func TestRangeFeedIntegration(t *testing.T) {
 		v1 := <-rows
 		require.Equal(t, mkKey("a"), v1.Key)
 		// Ensure the initial scan contract is fulfilled when WithDiff is specified.
-		require.Equal(t, v1.Value, v1.PrevValue)
+		require.Equal(t, v1.Value.RawBytes, v1.PrevValue.RawBytes)
 		require.Equal(t, v1.Value.Timestamp, afterB)
+		require.True(t, v1.PrevValue.Timestamp.IsEmpty())
 	}
 	{
 		v2 := <-rows

pkg/roachpb/api.proto

@@ -2385,6 +2385,7 @@ message RangeFeedValue {
   // 1. with_diff was passed in the corresponding RangeFeedRequest.
   // 2. the key-value was present and not a deletion tombstone before
   //    this event.
+  // The timestamp on the previous value is empty.
   Value prev_value = 3 [(gogoproto.nullable) = false];
 }