Make rootId param optional

[fremail]

Actually we need rootId param only for multi-root trees.

Here is a list of API functions with the issue:

• Model.fetchRoot = async function (rootId = 1, options = {})
• Model.fetchTree = async function (depth = 0, rootId = 1, options = {})

Also several private functions:

• Model.prototype.shiftRlValues = async function (first, delta, rootId = 1, options = {})
• Model.prototype.shiftRlRange = async function (first, last, delta, rootId = 1, options = {})