DittoLog

Main singleton (global instance) to use / call into ditto_log and thus our logging API.

It is expected to be an internal one, although it cannot be marked as such since it is used by other packages (e.g., ditto, androidtransports).

Tweaking its behavior, however (custom log cb, log file, log level, etc.), is intended to be public / customer facing. That part is thus defined as being part of ditto (our current public-facing package), as the DittoLogging singleton.