Switch from logger to log context - part 1:

- define a new struct jk_log_context_t,
  containing a jk_logger_t and a log context
  id, which can be used eg. by a request id

- add jk_log_content_t to the service struct

- change function declarations from using a
  jk_logger_t argument to jk_log_context_t

This breaks compilation, but the huge change is split up in parts
to simplify review.
17 files changed