42711 multiple log messages

- Bump Cocoalumberjack to version 2.0.0-rc
- Set up test logging inside dispatch_once block
Logs should be off during most test runs because they're not useful for successful runs, take up disk space and slow down tests. We can re-create failed runs on our development machines. If we can't, then that's something that needs fixing.

Tests shouldn't need to clean up after themselves wherever possible -- an exception during is likely to leave things behind, thereby contaminating other tests. This is why teardown cleans up the folder where databases live (at least hopefully XCTest calls it...). Cleaning up remote data is more difficult, hence the random suffix on the database name.

Speaking of async logging, it seems like perhaps logging should be sync during (local) testing, so we can be sure logs written appear before the test harness is torn down.

@tomblench what do you think here?