northd: Track ovn_datapaths in northd engine track data. #3444

Job	Run time
linux rpm fedora	12m 43s
osx clang --disable-ssl	5m 21s
dpdk gcc	3m 14s
linux gcc --disable-ssl	3m 13s
linux clang --disable-ssl	3m 5s
linux gcc test -300	11m 36s
linux gcc test 301-600	8m 57s
linux gcc test 601-	6m 29s
linux clang test sanitizers -300	19m 28s
linux clang test sanitizers 301-600	12m 31s
linux clang test sanitizers 601-	9m 3s
linux gcc test -ljemalloc -300	15m 13s
linux gcc test -ljemalloc 301-600	9m 33s
linux gcc test -ljemalloc 601-	4m 45s
linux gcc system-test-dpdk dpdk -100	16m 40s
linux gcc system-test-dpdk dpdk 101-	14m 36s
linux gcc system-test-userspace -100	13m 30s
linux gcc system-test-userspace 101-	12m 47s
linux gcc system-test -100	16m 28s
linux gcc system-test 101-	14m 34s
linux clang system-test sanitizers -100	17m 55s
linux clang system-test sanitizers 101-	15m 59s
linux x86 gcc --disable-ssl	3m 9s
	4h 10m 49s

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