ofproto-dpif: Improve load balancing in dp_hash select groups. #3099

Job	Run time
linux deb no dpdk	4m 58s
osx clang --disable-ssl	1m 26s
dpdk gcc	1m 53s
linux gcc --disable-ssl	2m 16s
linux clang --disable-ssl	2m 5s
linux gcc c99	1m 55s
linux clang c99	2m 5s
linux gcc test	7m 22s
linux clang test asan	12m 14s
linux clang test ubsan	8m 56s
linux gcc test --enable-shared	9m 56s
linux clang test --enable-shared	10m 14s
linux gcc test dpdk	7m 36s
linux clang test dpdk	8m 48s
linux gcc test -ljemalloc	8m 32s
linux clang test -ljemalloc	7m 50s
linux gcc --enable-afxdp	2m 4s
linux clang --enable-afxdp	2m 1s
linux gcc dpdk --enable-shared	2m 57s
linux clang dpdk --enable-shared	3m 4s
linux gcc dpdk-shared	2m 18s
linux clang dpdk-shared	2m 13s
linux gcc dpdk-shared --enable-shared	3m 0s
linux clang dpdk-shared --enable-shared	2m 48s
linux gcc m32 --disable-ssl	1m 59s
	2h 0m 30s

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