DSL to describe the "update" clause for an in place update

[sullivan-]

This is prerequisite for any further updateByKeyVal or updateByQuery work.

• I would recommend keeping it simple at first.
• Follow longevity.model.query.QueryFilter as a rough model.
• Basic structure is <prop> 'gets' <update-expr>
  • Provide both an operator (:= maybe) and a pneumonic equivalent such as gets)
• It's probably a good idea to restrict it to basic properties for the time being (i.e., things like ints and strings, not things like nested components, sets and lists)
• The update expression will vary by type; LHS and RHS have to match on type
  • update expression should allow for literals, basic properties, and basic operators over the types such as addition, logical or, string concatenation, etc.
• Please do write some unit tests to make sure the DSL is fluent
• When in doubt, ask @sullivan- he will be happy to help

In the future, we might add support for things like:

• collections and nested components
• specialized operators such as incr that can either rely on underlying DB operators such as MongoDB's $inc, or be expanded into equivalent expression e.g. x + 1