Added a new field `service` for serverStatus.

With PM-635, shards will be able to act as routers, and it is not enough to inform that the process is 'mongod' by serverStatus.process. In order to add extra information on the role that the mongod is acting at that moment, we have added a new field called `service`.

Now serverStatus returns

{ process: mongod, service: router|shard, ...}

running it directly to the mongod, or

{ process: mongos, service: router, ...}

running it through the mongos.

Description of Linked Ticket

As of build 7.3.0-alpha0-81-g99853e7 (which corresponds to git hash 99853e7a9c71d793b5b3c5028e322bf0215d2697), running db.serverStatus() against an embedded router returns process: 'mongod'. While this is technically correct, it'd be helpful to have a flag somewhat along the lines of service: 'router' or something similar to signify that this command was run against a router and not against the mongod directly.