Make DPanic an actual level and replace DFatal

[jcorbin]

DFatal not being an explicit log level, but instead an implementation detail of Meta, hurts composability:

• as mentioned on Next Steps wrt level #166
• as discussed during review of Refactor sampler checking #172 (this is the requested tracking issue)

[jcorbin]

My proposal:

• add DFatalLevel before PanicLevel, or before FatalLevel (putting it after FatalLevel requires more code change, as FatalLevel is assumed maximal; yes that means we're missing a semantic name for "maximal logging level", but that's another issue ;-) )
• add a Meta.DFatal(log, msg, fields) convenience method that can be used to ease implementation (e.g. func (log TConcreteLogger) DFatal(...) { log.Meta.DFatal(log, ...) }), all it will do is delegate to logger.Error or logger.Fatal depending on Meta.Development

[akshayjshah]

@jcorbin Plan makes sense, but let's replace DFatal with DPanic. It accomplishes the same blow-up-in-dev goal (and actually dumps a stack trace with the panic), and it fits more neatly into the level hierarchy between ErrorLevel and PanicLevel.