northd: Fix ecmp group selection by route source. #3397

Job	Run time
Prepare container	51s
linux rpm fedora	7m 20s
osx clang --disable-ssl	2m 2s
linux gcc --disable-ssl	2m 42s
linux clang --disable-ssl	2m 48s
linux gcc test -300	9m 55s
linux gcc test 301-600	7m 12s
linux gcc test 601- unstable	7m 27s
linux clang test sanitizers -300	13m 44s
linux clang test sanitizers 301-600	10m 9s
linux clang test sanitizers 601- unstable	12m 59s
linux gcc test -ljemalloc -300	10m 37s
linux gcc test -ljemalloc 301-600	7m 30s
linux gcc test -ljemalloc 601- unstable	7m 45s
linux gcc system-test-dpdk dpdk -100	17m 20s
linux gcc system-test-dpdk dpdk 101- unstable	17m 28s
linux gcc system-test-userspace -100	16m 25s
linux gcc system-test-userspace 101- unstable	16m 6s
linux gcc system-test -100	18m 42s
linux gcc system-test 101- unstable	20m 45s
linux clang system-test sanitizers -100	21m 8s
linux clang system-test sanitizers 101- unstable	22m 53s
linux x86 gcc --disable-ssl	2m 33s
	4h 16m 21s

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