New logging API

[snej]

API:

• Implemented new C4LogObserver API as described in the design doc.
• It was necessary to change some includes: c4Base.h no longer includes c4Log.h, it's now the other way around. Platform code may need to add some explicit #include c4log.h statements.
• The existing API for logging callbacks and files still exists, but should eventually be deprecated / removed.
• There is no logging by default: something has to register an observer. c4log_initConsole() adds an observer that logs to the console. CppTests and C4Tests call it at startup.

Implementation:

• Lots of refactoring of logging code. New class LogObserver is used to receive log messages.
• File and callback logging are implemented as subclasses LogFiles and LogCallback.

Review notes:

• The first three commits are just some preparations, moving the c4log code and moving the logging files to a new dir.
• "Implemented LogObserver. Refactored logging code." is the internal implementation, adding the LogObserver class and moving the file and callback logging code into implementations of it.
• "Implemented C4LogObserver [API]" implements the C API for LogObserver.
• "Heavily refactored LogFiles" is where I succumbed to the unnecessary task of cleaning up the file logging code. I merged the three parallel arrays (of file streams, log encoders and rotation counts) into an array of a new LogFile class, and most of the logic went into the new class.
• "Reimplemented old c4log API using new one" simplifies the implementation of the old C file/callback log APIs by calling the C LogObserver APIs.
• "c4Tests' logging stub should preserve the domain" just fixes a small mistake in the C4Tests logging glue: it should preserve the log domain instead of routing the logs to the default logger. This also eliminates the lint warning that the method could be made static.

[cbl-bot]

Code Coverage Results:

Type	Percentage
branches	66.19
functions	78.56
instantiations	70.78
lines	77.59
regions	73.39