northd: Avoid most of strcmp for NAT type. #1425

Job	Run time
linux rpm fedora	6m 54s
Prepare container	1m 12s
osx clang --disable-ssl	2m 28s
linux gcc --disable-ssl	2m 39s
linux clang --disable-ssl	2m 36s
linux gcc test -300	9m 15s
linux gcc test 301-600	6m 26s
linux gcc test 601- unstable	6m 38s
linux clang test sanitizers -300	12m 32s
linux clang test sanitizers 301-600	8m 59s
linux clang test sanitizers 601- unstable	10m 52s
linux gcc test -ljemalloc -300	9m 50s
linux gcc test -ljemalloc 301-600	6m 41s
linux gcc test -ljemalloc 601- unstable	6m 37s
linux gcc system-test-dpdk dpdk -100	17m 2s
linux gcc system-test-dpdk dpdk 101- unstable	14m 10s
linux gcc system-test-userspace -100	16m 0s
linux gcc system-test-userspace 101- unstable	13m 4s
linux gcc system-test -100	18m 35s
linux gcc system-test 101- unstable	16m 55s
linux clang system-test sanitizers -100	20m 24s
linux clang system-test sanitizers 101- unstable	18m 33s
linux x86 gcc --disable-ssl	2m 44s
	3h 51m 6s

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