Tree mode optimization

[YuriGor]

Now in the tree mode deepdash walks over all the nested fields and checks each path of it matches given 'children' criteria, because children path may be configured as a regexp, and it's impossible to say beforehand which nested value is a children collection, until we build and tested it's path.

Regexp support in the 'children' option should be dropped as an overkill. If only constant paths will be allowed, we will be able to go and get children collection directly, without bruteforcing all the possible variants.