northd: Add ipv6_{src, dst} to selection_fields column in the NB db. #4699

Job	Run time
linux rpm fedora	7m 21s
osx clang --disable-ssl	1m 52s
Prepare container	3m 43s
linux gcc --disable-ssl	2m 39s
linux clang --disable-ssl	2m 40s
linux gcc test -300	9m 47s
linux gcc test 301-600	7m 6s
linux gcc test 601- unstable	6m 41s
linux clang test sanitizers -300	12m 45s
linux clang test sanitizers 301-600	9m 34s
linux clang test sanitizers 601- unstable	11m 36s
linux gcc test -ljemalloc -300	9m 55s
linux gcc test -ljemalloc 301-600	6m 45s
linux gcc test -ljemalloc 601- unstable	6m 49s
linux gcc system-test-dpdk dpdk -100	17m 8s
linux gcc system-test-dpdk dpdk 101- unstable	14m 31s
linux gcc system-test-userspace -100	16m 8s
linux gcc system-test-userspace 101- unstable	13m 10s
linux gcc system-test -100	18m 20s
linux gcc system-test 101- unstable	17m 23s
linux clang system-test sanitizers -100	20m 15s
linux clang system-test sanitizers 101- unstable	20m 12s
linux x86 gcc --disable-ssl	2m 37s
	3h 58m 57s

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