northd: Fix recompute of referenced chassis in HA chassis groups. #150

Job	Run time
osx clang --disable-ssl	4m 50s
linux gcc --disable-ssl	3m 24s
linux clang --disable-ssl	4m 14s
linux gcc test -500	23m 25s
linux gcc test 501-1000	13m 11s
linux gcc test 1001-	6m 38s
linux clang test sanitizers -300	17m 37s
linux clang test sanitizers 301-600	24m 29s
linux clang test sanitizers 601-900	12m 22s
linux clang test sanitizers 901-1200	10m 10s
linux clang test sanitizers 1201-	8m 21s
linux gcc test -ljemalloc -500	28m 1s
linux gcc test -ljemalloc 501-1000	16m 44s
linux gcc test -ljemalloc 1001-	7m 6s
linux clang test -ljemalloc -500	22m 49s
linux clang test -ljemalloc 501-1000	13m 52s
linux clang test -ljemalloc 1001-	7m 24s
linux gcc system-test -100	17m 21s
linux gcc system-test 101-200	21m 42s
linux gcc system-test 201-	20m 38s
linux clang system-test sanitizers -100	19m 58s
linux clang system-test sanitizers 101-200	23m 48s
linux clang system-test sanitizers 201-	21m 35s
linux x86 gcc --disable-ssl	4m 26s
	5h 54m 5s

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