northd: Handle load balancer changes for a logical switch. #506

Job	Run time
osx clang --disable-ssl	5m 40s
dpdk gcc	3m 29s
linux rpm fedora	10m 13s
linux gcc --disable-ssl	2m 41s
linux clang --disable-ssl	3m 10s
linux gcc test -500	26m 25s
linux gcc test 501-1000	13m 31s
linux gcc test 1001-	5m 51s
linux clang test sanitizers -300	15m 45s
linux clang test sanitizers 301-600	23m 45s
linux clang test sanitizers 601-900	15m 25s
linux clang test sanitizers 901-1200	10m 33s
linux clang test sanitizers 1201-	7m 58s
linux gcc test -ljemalloc -500	24m 9s
linux gcc test -ljemalloc 501-1000	16m 54s
linux gcc test -ljemalloc 1001-	8m 25s
linux gcc system-test-dpdk dpdk -100	16m 23s
linux gcc system-test-dpdk dpdk 101-200	21m 21s
linux gcc system-test-dpdk dpdk 201-	23m 2s
linux gcc system-test-userspace -100	14m 55s
linux gcc system-test-userspace 101-200	19m 23s
linux gcc system-test-userspace 201-	22m 54s
linux gcc system-test -100	16m 24s
linux gcc system-test 101-200	21m 11s
linux gcc system-test 201-	25m 38s
linux clang system-test sanitizers -100	17m 5s
linux clang system-test sanitizers 101-200	24m 28s
linux clang system-test sanitizers 201-	30m 48s
linux x86 gcc --disable-ssl	2m 39s
	7h 30m 5s

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