northd: Don't SNAT reply packets on LBs with lb_force_snat_ip set. #4701

Job	Run time
osx clang --disable-ssl	1m 56s
Prepare container	3m 41s
linux rpm fedora	6m 53s
linux gcc --disable-ssl	2m 51s
linux clang --disable-ssl	2m 36s
linux gcc test -300	9m 24s
linux gcc test 301-600	6m 21s
linux gcc test 601- unstable	6m 31s
linux clang test sanitizers -300	12m 47s
linux clang test sanitizers 301-600	9m 9s
linux clang test sanitizers 601- unstable	11m 18s
linux gcc test -ljemalloc -300	9m 56s
linux gcc test -ljemalloc 301-600	6m 34s
linux gcc test -ljemalloc 601- unstable	7m 4s
linux gcc system-test-dpdk dpdk -100	17m 1s
linux gcc system-test-dpdk dpdk 101- unstable	14m 33s
linux gcc system-test-userspace -100	16m 3s
linux gcc system-test-userspace 101- unstable	13m 17s
linux gcc system-test -100	18m 23s
linux gcc system-test 101- unstable	17m 44s
linux clang system-test sanitizers -100	20m 20s
linux clang system-test sanitizers 101- unstable	19m 28s
linux x86 gcc --disable-ssl	2m 58s
	3h 56m 48s

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