northd: Introduce ECMP_Nexthop table in SB db. #4759

Job	Run time
Prepare container	3m 14s
linux rpm fedora	19m 23s
osx clang --disable-ssl	2m 5s
linux gcc --disable-ssl	2m 45s
linux clang --disable-ssl	2m 51s
linux gcc test -300	9m 22s
linux gcc test 301-600	6m 33s
linux gcc test 601- unstable	6m 35s
linux clang test sanitizers -300	12m 51s
linux clang test sanitizers 301-600	9m 37s
linux clang test sanitizers 601- unstable	12m 0s
linux gcc test -ljemalloc -300	9m 53s
linux gcc test -ljemalloc 301-600	6m 52s
linux gcc test -ljemalloc 601- unstable	6m 50s
linux gcc system-test-dpdk dpdk -100	17m 6s
linux gcc system-test-dpdk dpdk 101- unstable	14m 57s
linux gcc system-test-userspace -100	15m 58s
linux gcc system-test-userspace 101- unstable	13m 48s
linux gcc system-test -100	18m 33s
linux gcc system-test 101- unstable	18m 6s
linux clang system-test sanitizers -100	20m 17s
linux clang system-test sanitizers 101- unstable	20m 1s
linux x86 gcc --disable-ssl	2m 32s
	4h 12m 9s

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