northd: Don't SNAT reply packets on LBs with lb_force_snat_ip set. #2551

Job	Run time
dpdk gcc	4s
osx clang --disable-ssl	1m 50s
Prepare container	2m 21s
linux rpm fedora	6m 32s
linux gcc --disable-ssl	2m 17s
linux clang --disable-ssl	2m 28s
linux gcc test -300	9m 4s
linux gcc test 301-600	5m 34s
linux gcc test 601- unstable	6m 9s
linux clang test sanitizers -300	11m 46s
linux clang test sanitizers 301-600	7m 58s
linux clang test sanitizers 601- unstable	9m 37s
linux gcc test -ljemalloc -300	9m 28s
linux gcc test -ljemalloc 301-600	5m 47s
linux gcc test -ljemalloc 601- unstable	6m 10s
linux gcc system-test-dpdk dpdk -100	15m 56s
linux gcc system-test-dpdk dpdk 101- unstable	11m 11s
linux gcc system-test-userspace -100	15m 0s
linux gcc system-test-userspace 101- unstable	10m 8s
linux gcc system-test -100	17m 34s
linux gcc system-test 101- unstable	13m 59s
linux clang system-test sanitizers -100	19m 19s
linux clang system-test sanitizers 101- unstable	15m 53s
linux x86 gcc --disable-ssl	2m 12s
	3h 28m 17s

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