Change type of "_id" field of replica set config to be different from that of the member index

Original Description:

Replica set config documents look like:

{

   _id: 'rs name',

   members: [

      {_id: 0, host: 'host1:27017'},

      {_id: 1, host: 'host2:27017'}

   ]

}

The "_id" field in member subdocuments isn't useful in any way I know, but requires extra typing and it must be chosen to obey certain rules (it must be unique and between 0 and 255 inclusive). If we handle it incorrectly it can cause bugs (SERVER-40355). In summary, the _id field has cost but perhaps no benefit: uniqueness is provided by members' addresses and order is provided by their array position.

Let's consider deprecating _id, and eventually ignoring it completely.

New Description:
Since this has diagnostic value, we'd prefer to change the type of the "_id" field to avoid introducing bugs.