Enable 'allowDiskUse' for aggregations. Without this, errors ensue

allowDiskUse is required for aggregations that can't be completed completely in memory. This will be a common occurrence on large data sets.

Without this, we experience errors such as: `exception: Exceeded memory limit for $group, but didn't allow external sort. Pass allowDiskUse:true to opt in.`

Need to use `mongo/command` directly as the `aggregate` api does not expose options

`sorted-map` is required because mongo commands require an object whose first key is the command to be executed.