The simple_filter macro must accept the two different levels of
contexts. In the same way, it should also accept the log level, and
therefore replace the now removed simple_filter_log_level macro.
Threads are a bad idea because for now the filter API is not guaranteed
to be state-less. The interface is now synchronous, which should be
enough for most filters.
The refactoring brought other changes, the most important being the
concept of modular input sources and output destination and the complete
rewrite of the procedural macro.