northd: Don't SNAT reply packets on LBs with lb_force_snat_ip set. #1485

Job	Run time
linux rpm fedora	7m 14s
osx clang --disable-ssl	2m 8s
Prepare container	3m 31s
linux gcc --disable-ssl	2m 37s
linux clang --disable-ssl	2m 37s
linux gcc test -300	9m 15s
linux gcc test 301-600	6m 31s
linux gcc test 601- unstable	5m 16s
linux clang test sanitizers -300	12m 35s
linux clang test sanitizers 301-600	9m 1s
linux clang test sanitizers 601- unstable	8m 47s
linux gcc test -ljemalloc -300	9m 50s
linux gcc test -ljemalloc 301-600	6m 40s
linux gcc test -ljemalloc 601- unstable	5m 20s
linux gcc system-test-dpdk dpdk -100	16m 52s
linux gcc system-test-dpdk dpdk 101- unstable	14m 23s
linux gcc system-test-userspace -100	16m 8s
linux gcc system-test-userspace 101- unstable	13m 12s
linux gcc system-test -100	18m 19s
linux gcc system-test 101- unstable	17m 32s
linux clang system-test sanitizers -100	20m 16s
linux clang system-test sanitizers 101- unstable	19m 17s
linux x86 gcc --disable-ssl	2m 37s
	3h 49m 58s

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