reduce number of DHCP responder flows for LSPs #2394

Job	Run time
dpdk gcc	9s
linux rpm fedora	6m 48s
osx clang --disable-ssl	7m 55s
linux gcc --disable-ssl	1m 11s
linux clang --disable-ssl	1m 17s
linux gcc test -300	8m 31s
linux gcc test 301-600	6m 7s
linux gcc test 601-	4m 3s
linux clang test sanitizers -300	11m 30s
linux clang test sanitizers 301-600	8m 45s
linux clang test sanitizers 601-	6m 38s
linux gcc test -ljemalloc -300	8m 50s
linux gcc test -ljemalloc 301-600	6m 10s
linux gcc test -ljemalloc 601-	4m 6s
linux gcc system-test-dpdk dpdk -100	1m 18s
linux gcc system-test-dpdk dpdk 101-	1m 20s
linux gcc system-test-userspace -100	1m 9s
linux gcc system-test-userspace 101-	1m 10s
linux gcc system-test -100	1m 11s
linux gcc system-test 101-	1m 8s
linux clang system-test sanitizers -100	1m 19s
linux clang system-test sanitizers 101-	1m 21s
linux x86 gcc --disable-ssl	1m 10s
	1h 33m 6s

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