northd: Fix non-functional incremental processing of port groups. #216

Job	Run time
linux rpm fedora	7m 18s
osx clang --disable-ssl	2m 2s
Prepare container	3m 50s
linux gcc --disable-ssl	2m 39s
linux clang --disable-ssl	2m 38s
linux gcc test -300	9m 31s
linux gcc test 301-600	6m 30s
linux gcc test 601- unstable	6m 19s
linux clang test sanitizers -300	12m 35s
linux clang test sanitizers 301-600	9m 12s
linux clang test sanitizers 601- unstable	10m 34s
linux gcc test -ljemalloc -300	9m 43s
linux gcc test -ljemalloc 301-600	6m 41s
linux gcc test -ljemalloc 601- unstable	6m 26s
linux gcc system-test-dpdk dpdk -100	17m 45s
linux gcc system-test-dpdk dpdk 101- unstable	13m 59s
linux gcc system-test-userspace -100	15m 41s
linux gcc system-test-userspace 101- unstable	17m 20s
linux gcc system-test -100	18m 22s
linux gcc system-test 101- unstable	16m 31s
linux clang system-test sanitizers -100	20m 14s
linux clang system-test sanitizers 101- unstable	19m 28s
linux x86 gcc --disable-ssl	2m 36s
	3h 57m 54s

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