Create a unified Snuba consumer

[BYK]

Currently we have many Snuba consumers and seems like we are going to add more as we add more features around Snuba. This poses a maintenance and synchronization issue for on-premise as the growing number of consumers is hard to maintain. People also keep forgetting to add new consumers to the self-hosted repo, causing issues or missing features (this has happened twice now).

Proposal: introduce a consumers command to Snuba CLI that runs all core consumers with default settings (the settings we use in self-hosted) using multiprocessing so the consumers are still logically separate.

/cc @fpacifici @lynnagara @chadwhitacre

[BYK]

[chadwhitacre]

Here it is @JoshFerge. :)