Easy syntax for $setIfModified behavior

see SERVER-13578. This behavior is "possible" using update pipeline syntax released in 4.2, but it becomes extremely verbose for this common use case.

Example 

db.runCommand(

 {

 update: "test",

 updates: [

 {

 q: { a: 1 },

 u: [

 { $set: { a: 1, b: 2,

created_at: {$cond: [

 {$eq:[ {$type:"$_id"}, "missing" ] },

 ISODate(),

 "$created_at"

 ] },

updated_at: {$cond: [

 {$or: [

 $ne:["$a", 1],

 $ne:["$b", 2],

 ]},

 ISODate(),

 "$updated_at"

 ] }

} },

 ],

 upsert: true

 }

 ],

 writeConcern: { w: "majority", wtimeout: 5000 }

 }

)

You can see that as the complexity of the document grows, the more verbose the command becomes to get this behavior. It would be nice to get this behavior with an easy to use operator (e.g. $setIfModified)