zebra: Cleanup ctx leak on shutdown and turn off event

zebra: Cleanup ctx leak on shutdown and turn off event

two things:

On shutdown cleanup any events associated with the update walker.
Also do not allow new events to be created.

Fixes this mem-leak:

./msdp_topo1.test_msdp_topo1/r2.zebra.asan.1117790:Direct leak of 8 byte(s) in 1 object(s) allocated from:
./msdp_topo1.test_msdp_topo1/r2.zebra.asan.1117790-    #0 0x7f0dd0b08037 in __interceptor_calloc ../../../../src/libsanitizer/asan/asan_malloc_linux.cpp:154
./msdp_topo1.test_msdp_topo1/r2.zebra.asan.1117790-    #1 0x7f0dd06c19f9 in qcalloc lib/memory.c:105
./msdp_topo1.test_msdp_topo1/r2.zebra.asan.1117790-    #2 0x55b42fb605bc in rib_update_ctx_init zebra/zebra_rib.c:4383
./msdp_topo1.test_msdp_topo1/r2.zebra.asan.1117790-    #3 0x55b42fb6088f in rib_update zebra/zebra_rib.c:4421
./msdp_topo1.test_msdp_topo1/r2.zebra.asan.1117790-    #4 0x55b42fa00344 in netlink_link_change zebra/if_netlink.c:2221
./msdp_topo1.test_msdp_topo1/r2.zebra.asan.1117790-    #5 0x55b42fa24622 in netlink_information_fetch zebra/kernel_netlink.c:399
./msdp_topo1.test_msdp_topo1/r2.zebra.asan.1117790-    #6 0x55b42fa28c02 in netlink_parse_info zebra/kernel_netlink.c:1183
./msdp_topo1.test_msdp_topo1/r2.zebra.asan.1117790-    #7 0x55b42fa24951 in kernel_read zebra/kernel_netlink.c:493
./msdp_topo1.test_msdp_topo1/r2.zebra.asan.1117790-    #8 0x7f0dd0797f0c in event_call lib/event.c:1995
./msdp_topo1.test_msdp_topo1/r2.zebra.asan.1117790-    #9 0x7f0dd0684fd9 in frr_run lib/libfrr.c:1185
./msdp_topo1.test_msdp_topo1/r2.zebra.asan.1117790-    #10 0x55b42fa30caa in main zebra/main.c:465
./msdp_topo1.test_msdp_topo1/r2.zebra.asan.1117790-    #11 0x7f0dd01b5d09 in __libc_start_main ../csu/libc-start.c:308
./msdp_topo1.test_msdp_topo1/r2.zebra.asan.1117790-
./msdp_topo1.test_msdp_topo1/r2.zebra.asan.1117790-SUMMARY: AddressSanitizer: 8 byte(s) leaked in 1 allocation(s).

Signed-off-by: Donald Sharp <sharpd@nvidia.com>
(cherry picked from commit 3cd0accb5051e900ab343d5fa7c59360c0c1a812)
Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>