Use a single, non-replaceable default logger

[jonyoder]

Connected to #22. This PR:

• Removes the SetDefaultLogger method. Instead of replacing the default logger with a new reference, simply use the Set* methods to update the existing default logger as needed.
• Adds the methods SetFormatter and Tracef to the logger interface.
• Adds the method Tracef to the debug logger interface.
• Adds a public global DefaultLoggerFactory that can be set in an init() method to provide the rslog package with a different default logger. If this is unset, rslog will automatically create its own factory and default logger. See root.go in the example command directory for an example.
• Removes the logger interface's Copy method. We no longer support copying a logger since it makes SIGHUP support impossible.
• Removes the logger interface's SetReportCaller method. Since logrus reports the first non-logrus line of code, this feature was useless since it always simply reported a line of code in the rslog package.
• Updates the debug logger to use the underlying defaultLogger rather than copying an existing logger.
• Updates the example command with a debug command and a new global --level flag.
• Adds a Compare(l LogLevel) method to the LogLevel struct so you can compare log levels based on the underlying logrus values.

Using the example commands:

just build
./out/testapp debug --message="hi jonathan" --level=TRACE
./out/testapp debug --message="hi jonathan" --level=DEBUG
./out/testapp log --message="hi jonathan" --level=INFO
./out/testapp log --message="hi jonathan" --level=WARN

[marcosnav]

We'll want to bump a new version of the package when this changes land. But before that, we also want to rename rslog/loggertest to rslog/rslogtest. It would be good to push both things in the same tag. 🙂

[jonyoder]

I will make the change to rslogtest in this PR while I'm here.

[jonyoder]

Also added one more commit that includes an example service you can run to prove that logger reconfiguration works: 72b5b6a.

[jonyoder]

@marcosnav @eduardocfalcao Are you ok with merging this one now?

[marcosnav]

@marcosnav @eduardocfalcao Are you ok with merging this one now?

@jonyoder Yes, I'm okay with it :)