Expire tracking after 30ish days

[Martii]

• We really don't need to keep this info around forever.
• Indicate in nomenclature

Applies to #1730

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Needs mitigation added ... see Automattic/mongoose#7703 (comment) and forward

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I went ahead and dropped the index name and now it's at 2592000 with db.syncs.getIndexes(); with a server restart. Still doesn't explain why it was set to 7d when it's never seen that value. shrugs

Ref:

> db.syncs.getIndexes();
[
        {
                "v" : 2,
                "key" : {
                        "_id" : 1
                },
                "name" : "_id_"
        },
        {
                "v" : 2,
                "key" : {
                        "created" : 1
                },
                "name" : "created_1",
                "expireAfterSeconds" : 604800,
                "background" : true
        }
]
> db.syncs.dropIndex('created_1', function(err, result) {});
{ "nIndexesWas" : 2, "ok" : 1 }
> db.syncs.getIndexes();
[ { "v" : 2, "key" : { "_id" : 1 }, "name" : "_id_" } ]

... restart app.js ...

> db.syncs.getIndexes();
[
        {
                "v" : 2,
                "key" : {
                        "_id" : 1
                },
                "name" : "_id_"
        },
        {
                "v" : 2,
                "key" : {
                        "created" : 1
                },
                "name" : "created_1",
                "expireAfterSeconds" : 2592000,
                "background" : true
        }
]

NOTE:

• Never use the shorthand property (7d) because it can be cast to a 32 bit Number in some packages and possibly native JavaScript.