Move logging of time measurements to FINE level

We've been getting user feedback where most of the log is timing measurements (e.g. when loading a private conversation with 100 messages), so more important log messages get pushed out.

This branch moves the logging of time measurements to the FINE level, and sets the default log level to FINE for debug and beta builds and INFO for release builds. System.nanoTime() is used instead of System.currentTimeMillis() for timing measurements because it's usually much cheaper to call.