Naming changes to $indexStats aggregation stage output format

Changes under this ticket will cover the following changes to the $indexStats aggregation source output:

1. Removing namespace (which is redundant)
2. Name changes to several of the output fields to bring in line with server naming convention

The new output format will be as follows:

{

    "name" : "a_1",

    "host" : "foo:27017",

    "accesses" : {

        "ops" : NumberLong(5),

        "since" : ISODate("2015-09-21T14:11:56.095Z")

    }

}