Use mongo $bit operation instead of $inc for tracking agent liveness

[John A Meinel]

https://docs.mongodb.com/manual/reference/operator/update/bit/
Bitwise OR was supported all the way back to Mongo 2.2

We currently have to manage a whole "presence.beings" table to ensure that we never ping the same slot 2 times because we are using a $inc instead of a bitwise or.
However, bitwise or has been supported since Mongo 2.2. Which means we could just reuse any slot for a given operator instead of growing them without bound and having to prune them, etc.

We'd also need to know whether $inc vs $or is going to be a performance issue.

[John A Meinel (jameinel)]

https://github.com/juju/juju/pull/7863

addresses this and bug #1703675 at the same time.

[John A Meinel (jameinel)]

So this did land, but it turns out that Mongo 2.4 "supports" $bit. Specifically, in 2.4 it fails to create a document if you specify upsert:

3.2:
> db.foo.update({_id: "a"}, {$bit: {"a": {"or": NumberLong(1)}}}, {upsert: 1})
WriteResult({ "nMatched" : 0, "nUpserted" : 1, "nModified" : 0, "_id" : "a" })
> db.foo.find()
{ "_id" : "a", "a" : NumberLong(1) }
>

2.4:
> db.foo.update({_id: "a"}, {$bit: {"a": {"or": NumberLong(1)}}}, {upsert: 1})
WriteResult({ "nMatched" : 0, "nUpserted" : 0 })
> db.foo.find()

And since we still use 2.4 on Trusty, we either need to special case this, or not use $bit.

[John A Meinel (jameinel)]

This should have landed in 2.3-beta1