UI might not show some stats in mixed mode

In mixed mode UI uses old /_uistats to display stats. It turned out that /_uistats implementation assumes that atoms for stats names are created by the time the endpoint is used. If some atoms are not created UI will not show this stat. When we used old stats collectors (<= 6.5) they always created the atoms (during collections). Now we removed the old collectors, so some atoms might not be created by the time UI graphs are used, which leads to some graphs missing.