Flogger 0.6

• Eliminated self-reference in pom.xml. (6a468cc)
• Added gRPC context implementation. (79957c8)
• Renamed LoggingScope to LoggingContextCloseable. (8442b81)
• Tags no longer explicitly return a SortedSet/Map (callers should use <? extends Set/Map>). (b849fa9)
• Platform implementations now use ContextDataProvider instead of LoggingContext (which will be deleted soon).
• Removed JdkLoggerConfig (#142). (62d0db6)
• Added provisional support for metadata keys/values in scopes. (f2ab0fd)
• Made MetadataKey's emit() method properly typed for its value. (95ded18)
• Added with(MetadataKey<Boolean>) method for setting metadata "flags". (2f1243e)
• Improved the ScopedLoggingContext API. (169d67d)
• Added log source information to the Log4J2 Event. (e1c6794)
• Various refactorings and test improvements.