northd: Incrementally process SB.Load_balancer updates. #1609

Job	Run time
linux rpm fedora	12m 29s
dpdk gcc	7s
osx clang --disable-ssl	5m 25s
linux gcc --disable-ssl	2m 55s
linux clang --disable-ssl	2m 51s
linux gcc test -500	22m 45s
linux gcc test 501-1000	16m 8s
linux gcc test 1001-	7m 33s
linux clang test sanitizers -300	21m 10s
linux clang test sanitizers 301-600	22m 49s
linux clang test sanitizers 601-900	15m 20s
linux clang test sanitizers 901-1200	10m 14s
linux clang test sanitizers 1201-	8m 48s
linux gcc test -ljemalloc -500	24m 54s
linux gcc test -ljemalloc 501-1000	16m 54s
linux gcc test -ljemalloc 1001-	6m 48s
linux gcc system-test-dpdk dpdk -100	12m 47s
linux gcc system-test-dpdk dpdk 101-200	16m 25s
linux gcc system-test-dpdk dpdk 201-	23m 6s
linux gcc system-test-userspace -100	11m 19s
linux gcc system-test-userspace 101-200	16m 9s
linux gcc system-test-userspace 201-	19m 32s
linux gcc system-test -100	13m 34s
linux gcc system-test 101-200	17m 18s
linux gcc system-test 201-	24m 14s
linux clang system-test sanitizers -100	17m 14s
linux clang system-test sanitizers 101-200	19m 22s
linux clang system-test sanitizers 201-	28m 3s
linux x86 gcc --disable-ssl	2m 42s
	6h 58m 55s

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