New text logging format + moving initialization to new method

[isoos]

The new format is the combination of the current one (which blanks out the prefix on repeated lines) and pub-dev's worker format (which adds a timestamp in the prefix. It also makes the json logging simpler. The code refactor is in the second commit.

This is a less-important, but still required step before the pub worker can invoke the pana process without depending on it (while keeping the log format). As the next step, while the pub_worker still depends on pana as a library, it may use the separate method to initialize the logging.