Hidden Indexes (NODE-2591)

[reggi]

NODE-2591

PR to implement the feature "hidden indexes".

Uses a new HideIndexOperation to send the command specified in SPEC-1678's scope. This one operation handles both hiding indexes and unhiding index. I didn't see the point in creating two new operations when it's just a matter of adding a boolean option.

I didn't have access to the collection name within the operation so I added it in from the parent, this is because the ns replaces the collection with $cmd. Would love to know a better way of doing this if there is one.

Adds hidden option to createIndex and test for that as well.

I made an attempt at adding in the 'expireAfterSeconds' option to .hideIndex() but when sent I got this error:

Uncaught AssertionError: expected [MongoError: no expireAfterSeconds field to update] to not exist

So I figured it may not need to be added.

Adds hidden option to IndexOptions.

[reggi]

@mbroadst Can you give me some guidance to see if I'm on track here?

[mbroadst]

@reggi where did the requirement to make hideIndex/unhideIndex helpers come from? I'm not seeing that in the spec changes linked to the DRIVERS ticket. In adherence to the driver mantra minimize administrative helpers I would think we would not provide them.

The index management spec is focused on "immutable" interaction with indexes, even though the collMod method exists and allows indexes to be modified in place the goal of this spec change was to support a hidden property for index creation only.