bugfix: adjust the level of the root when deleting nodes

[tyrzhang]

bugfix: adjust the level of the root when deleting nodes

when the tree is deep, and deleting nodes, will cause the issue.
the tree could be like this: one obj but 3 levels depth.

{
  "size": 1,
  "depth": 3,
  "root": {
    "entries": [
      {
        "bb": "[1.00, 2.00]x[1.00, 2.00]",
        "child": {
          "entries": [
            {
              "bb": "[1.00, 2.00]x[1.00, 2.00]",
              "child": {
                "leaf": true,
                "entries": [
                  {
                    "bb": "[1.00, 2.00]x[1.00, 2.00]"
                  }
                ]
              }
            }
          ]
        }
      }
    ]
  }
}

so we need to merge the root in loop, instead of once.

[hanyuancheung]

We've met the same problem, @dhconnelly could you please help to review this PR, so we could fix the Production issue.
Thanks.

[dhconnelly]

looks great, thanks again!