Dots in Field names: Sorting, allow optional use of new Path expression (with escapes)

[tatu-at-datastax]

(note: part of #1664; requires implementation of #1851 first. And finally, requires handler for path expressions (issue to be filed))
(note similar to #1850 but for Sorting)

Although Sorting does not (and cannot) make use of new Path expression at this point (might in future), it'd be good to allow use of such expressions from beginning.
So, for document like:

{
  "price.usd": 20
}

user could use either of following references for filtering:

"sort" : {"price.usd": 1}

"sort" : {"price&.usd": 1}

and they would work the same, sorting documents like shown above.

That is, use of escape-by-ampersand is allowed even if not really used or required at this point. This also does not mean that the un-escaped form would be deprecated.