Keep trip state in one JSON file per route

[idreyn]

Resolves #59 (see that issue for a detailed description of the rationale)

This PR adds a trip_state.py that is responsible for keeping track of the current state of a given trip (i.e. the previous event that it recorded). The diff between the last event and the new one is used by process_event().

Instead of a single state.json file, we now write one trip_states/{route_id}.json file per route. This should increase disk write performance, with smaller files and no write contention between different event processing threads.

Test plan:

I ran python -m gobble and verified that:

• it runs correctly
• it writes files as expected to disk
• when I restart the server, the states are recovered from the json files

[devinmatte]

Also if you want to use datadog locally, you can set up a local agent with our account API keys and send profiles from local. Just set env=local or something

[idreyn]

(Sorry for the crazy number of commits from my other branch, they'll all get squashed down though)