pimd: rework MSDP mesh-group code

pimd: rework MSDP mesh-group code

Fully utilize the northbound to hold pointers to our private data
instead of searching for data structures every time we need to change a
configuration.

Highlights:

 * Support multiple mesh groups per PIM instance (instead of one)
 * Use DEFPY instead of DEFUN to reduce code complexity
 * Use northbound private pointers to store data structures
 * Reduce callback names size

Signed-off-by: Rafael Zalamena <rzalamena@opensourcerouting.org>