lib: add fine-grained debugging in the northbound

lib: add fine-grained debugging in the northbound

Split the "debug northbound" command into the following commands:
* debug northbound callbacks configuration
* debug northbound callbacks state
* debug northbound callbacks rpc
* debug northbound notifications
* debug northbound events
* debug northbound client confd
* debug northbound client sysrepo

If "debug northbound" is entered alone, all of its suboptions
are enabled.

This commit also adds code to debug state/rpc callbacks and
notifications (only configuration callbacks were logged before).

Use the debugging infrastructure from "lib/debug.h" in order to
benefit from its facilities (e.g. MT-safe debugging) and avoid
code duplication.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>