Implement "$inc" update operator

[tatu-at-datastax]

It looks like "$inc" operator has relatively simple limits (see https://www.mongodb.com/docs/manual/reference/operator/update/inc/ for full description):

• May be used on number-valued fields/elements
• May be used on missing (non-existing) fields/elements
• May NOT be used on non-number-valued elements (this includes null)

So it seems like a good candidate for implementation.

[tatu-at-datastax]

@vkarpov15 Does Mongoose use $inc extensively? I think implementation should be relatively straight-forward, but for priority/ordering it is good to know which features benefit project most.

[vkarpov15]

I added some notes here: https://gist.github.com/vkarpov15/d24c95f5590f8c4c3f92388646da01df .

A little more detail: yes, Mongoose does make heavy use of $inc by default. Specifically, Mongoose's default concurrency strategy for save() is a form of optimistic concurrency that kicks in only when there is a potentially conflicting array operation. For example, updating updates.0.quantity when someone else has removed updates.0. So, by default, adding an element to an array using push() and then calling save() triggers a $inc on the version key.

Because this behavior is the default and sufficiently unintrusive, most developers don't end up configuring the concurrency strategy. So most apps will end up using $inc under the hood.