Mongo integration

[avibryant]

Should be able to flush to mongo. For this, might want to be able to parse the monolithic simmer key into multiple fields on the mongo document, and similarly might want to store several aggregations in the same document. So, eg,

sum:country=US|age=30   2
min:country=US|age=30 1

might end up combining with:

{country: "US", age: 30, _sum: 20, _min: 1}