northd: Add northd change handler for sync_to_sb_lb node. #187

Job	Run time
dpdk gcc	2m 14s
osx clang --disable-ssl	4m 41s
linux rpm fedora	6m 44s
linux gcc --disable-ssl	1m 9s
linux clang --disable-ssl	1m 11s
linux gcc test -300	8m 36s
linux gcc test 301-600	5m 38s
linux gcc test 601-	3m 35s
linux clang test sanitizers -300	11m 25s
linux clang test sanitizers 301-600	8m 7s
linux clang test sanitizers 601-	5m 54s
linux gcc test -ljemalloc -300	8m 59s
linux gcc test -ljemalloc 301-600	5m 57s
linux gcc test -ljemalloc 601-	3m 40s
linux gcc system-test-dpdk dpdk -100	1m 18s
linux gcc system-test-dpdk dpdk 101-	1m 13s
linux gcc system-test-userspace -100	1m 4s
linux gcc system-test-userspace 101-	1m 4s
linux gcc system-test -100	1m 6s
linux gcc system-test 101-	1m 5s
linux clang system-test sanitizers -100	1m 22s
linux clang system-test sanitizers 101-	1m 21s
linux x86 gcc --disable-ssl	1m 5s
	1h 28m 28s

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