add tps report fetcher to diego_client
start pulling out instance reporting into an injected instances_reporter

Signed-off-by: Zach Robinson <zrobinson@pivotallabs.com>
fix spec - tell it how to new a DiegoClient
move healthy instances check in to instances_reporter
move find_crashes into instances_reporter
move find_stats call into instances_reporter
Centralize db connection logic

- So we can reintroduce some parallelism
- DB_CONNECTION was always sqlite:///... when unspecified, even if DB was set to mysql/postgres
- [#72550650]