Save verbose output to a log file

[chapulina]

The IGN_VERBOSE environment variable provides a way of getting useful information to debug a transport setup. It would be great if it would also save the logs to a file, for easier searching.

Desired behavior

Always log verbose output to a timestamped file under ~/.ignition/transport/log. It would be nice if the filename contained information about the process that generated the logs.

Alternatives considered

We could only save the log files when IGN_VERBOSE is set, for performance. But this means users have to remember to set the variable.

Implementation suggestion

It may be good for performance if we dumped to the file at once at the end of a run, instead of appending lines to the file as execution happens.

Another thing to consider is using ignition::common::Console, so the logs from transport end up together with other libraries' logs.

Additional context

Ignition Gazebo dumps logs to ~/.ignition/gazebo/log for every run, even if the user is not running in verbose mode. So choosing the verbose mode only affects what ends up in the console. All ignition::common::Console from upstream libraries end up in these logs.

Ignition Rendering dumps logs to ~/.ignition/rendering/<engine>.log, and the log is updated for every run. This saves space in the computer, but makes it inconvenient to compare several runs.