Avoid cache GC of writebacks before commit syncs (#13860)

Save part.1 for writebacks in a separate folder
and move it to cache dir atomically while saving
the cache metadata. This is to avoid GC mistaking
part.1 as orphaned cache entries and purging them.

This PR also fixes object size being overwritten during
retries for write-back mode.

cmd/disk-cache_test.go

@@ -25,7 +25,7 @@ import (
 // Tests ToObjectInfo function.
 func TestCacheMetadataObjInfo(t *testing.T) {
 	m := cacheMeta{Meta: nil}
-	objInfo := m.ToObjectInfo("testbucket", "testobject")
+	objInfo := m.ToObjectInfo()
 	if objInfo.Size != 0 {
 		t.Fatal("Unexpected object info value for Size", objInfo.Size)
 	}