DDB set support in updateExpression

[petermyers]

• Adds support for DDB set ADD and DELETE operations in updateExpression.
• Maintains order of SET / REMOVE operations to avoid breaking exists user tests.
• Set DELETE's handled via a 'special' _delete suffixed attribute since there's no real way of knowing otherwise what needs to be deleted.

[vsnig]

would be also nice to have an option to use functions such as if_not_exists(path, value) too

UpdateExpression: "SET #attr = if_not_exists(#attr, :val)"

Very useful for fields such as createdOn etc.