northd: Add option to enable conntrack for the LSP whose peer is l3dgw_port #2483

Job	Run time
linux rpm fedora	6m 31s
dpdk gcc	5s
osx clang --disable-ssl	4m 10s
linux gcc --disable-ssl	1m 51s
linux clang --disable-ssl	1m 57s
linux gcc test -300	8m 32s
linux gcc test 301-600	5m 27s
linux gcc test 601- unstable	5m 3s
linux clang test sanitizers -300	11m 19s
linux clang test sanitizers 301-600	8m 13s
linux clang test sanitizers 601- unstable	8m 37s
linux gcc test -ljemalloc -300	8m 51s
linux gcc test -ljemalloc 301-600	5m 32s
linux gcc test -ljemalloc 601- unstable	5m 6s
linux gcc system-test-dpdk dpdk -100	12m 49s
linux gcc system-test-dpdk dpdk 101- unstable	11m 57s
linux gcc system-test-userspace -100	11m 35s
linux gcc system-test-userspace 101- unstable	10m 52s
linux gcc system-test -100	14m 19s
linux gcc system-test 101- unstable	12m 24s
linux clang system-test sanitizers -100	15m 30s
linux clang system-test sanitizers 101- unstable	14m 16s
linux x86 gcc --disable-ssl	1m 50s
	3h 6m 46s

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