Better Logging

[dimitribouniol]

• Updated the main logger logic to use a fallback logger rather than a child logger, based on feedback from slack
• Renamed WebPushManager's logger to backgroundActivityLogger to match AsyncHTTPClient's use of a similar logger.
• Added loggers to the send methods.
• Added much more comprehensive logging metadata.
• Fixed the logging level of service logs to one appropriate for libraries.
• Fixed some missing backticks on documentation.