logtree: rename payload to leveledpayload

Test Plan: Refactor, covered by tests.

X-Origin-Diff: phab/D639
GitOrigin-RevId: 6c268c8b437a93c97720f110dbc9c39e95402648
diff --git a/core/pkg/logtree/payload_test.go b/core/pkg/logtree/payload_test.go
index 26219cd..789fbcf 100644
--- a/core/pkg/logtree/payload_test.go
+++ b/core/pkg/logtree/payload_test.go
@@ -18,8 +18,8 @@
 
 import "time"
 
-func testPayload(msg string) *Payload {
-	return &Payload{
+func testPayload(msg string) *LeveledPayload {
+	return &LeveledPayload{
 		message:   msg,
 		timestamp: time.Now(),
 		severity:  INFO,