zebra: use hooks for client lifecycle callbacks

zebra: use hooks for client lifecycle callbacks

zserv.c was using hardcoded callbacks to clean up various components
when a client disconnected. Ergo zserv.c had to know about all these
unrelated components that it should not care about. We have hooks now,
let's use the proper thing instead.

Signed-off-by: Quentin Young <qlyoung@cumulusnetworks.com>