Benchmark Multiraft.SubmitCommand.
Benchmark Multiraft.SubmitCommand.

```
make bench PKG=./multiraft TESTFLAGS='-benchmem' 2>/dev/null
go test -tags ''  -i ./multiraft
go test -tags ''  -run - -cpu 1 -bench . ./multiraft -timeout 5m -benchmem
PASS
BenchmarkSubmitCommand1Node1Pending 	  100000	     19396 ns/op	    2519 B/op	      30 allocs/op
BenchmarkSubmitCommand3Nodes1Pending	    5000	    326288 ns/op	   20528 B/op	     250 allocs/op
BenchmarkSubmitCommand5Nodes1Pending	    3000	    591363 ns/op	   37505 B/op	     453 allocs/op
BenchmarkSubmitCommand1Node2Pending 	  100000	     13240 ns/op	    1791 B/op	      23 allocs/op
BenchmarkSubmitCommand3Nodes2Pending	    5000	    280565 ns/op	   18846 B/op	     229 allocs/op
BenchmarkSubmitCommand5Nodes2Pending	    3000	    524631 ns/op	   35353 B/op	     425 allocs/op
BenchmarkSubmitCommand1Node4Pending 	  100000	     14925 ns/op	    1989 B/op	      24 allocs/op
BenchmarkSubmitCommand3Nodes4Pending	    5000	    260716 ns/op	   18823 B/op	     224 allocs/op
BenchmarkSubmitCommand5Nodes4Pending	    3000	    497292 ns/op	   35304 B/op	     418 allocs/op
ok  	github.com/cockroachdb/cockroach/multiraft	14.784s
```

cuongdo/cockroach,paperstreet/cockroach,paperstreet/cockroach,il9ue/cockroach,vivekmenezes/cockroach,sunya123/cockroach,veteranlu/cockroach,arjun4084346/cockroach,maxlang/cockroach,maxlang/cockroach,nieyy/cockroach,cuongdo/cockroach,glycerine/cockroach,il9ue/cockroach,petermattis/cockroach,maxlang/cockroach,petermattis/cockroach,cuongdo/cockroach,vivekmenezes/cockroach,glycerine/cockroach,arjun4084346/cockroach,petermattis/cockroach,cuongdo/cockroach,vaibhavarora/cockroach,vivekmenezes/cockroach,arjun4084346/cockroach,maxlang/cockroach,petermattis/cockroach,cuongdo/cockroach,cockroachdb/cockroach,veteranlu/cockroach,arjun4084346/cockroach,il9ue/cockroach,mbertschler/cockroach,petermattis/cockroach,cuongdo/cockroach,vivekmenezes/cockroach,il9ue/cockroach,maxlang/cockroach,petermattis/cockroach,paperstreet/cockroach,veteranlu/cockroach,mbertschler/cockroach,vaibhavarora/cockroach,vaibhavarora/cockroach,paperstreet/cockroach,nieyy/cockroach,arjun4084346/cockroach,marcuswestin/cockroach,marcuswestin/cockroach,paperstreet/cockroach,paperstreet/cockroach,petermattis/cockroach,petermattis/cockroach,vivekmenezes/cockroach,veteranlu/cockroach,vivekmenezes/cockroach,veteranlu/cockroach,bg451/cockroach,maxlang/cockroach,marcuswestin/cockroach,marcuswestin/cockroach,bg451/cockroach,glycerine/cockroach,bg451/cockroach,sunya123/cockroach,glycerine/cockroach,arjun4084346/cockroach,glycerine/cockroach,cockroachdb/cockroach,nieyy/cockroach,glycerine/cockroach,nieyy/cockroach,marcuswestin/cockroach,mbertschler/cockroach,cockroachdb/cockroach,paperstreet/cockroach,nieyy/cockroach,mbertschler/cockroach,veteranlu/cockroach,sunya123/cockroach,sunya123/cockroach,vaibhavarora/cockroach,marcuswestin/cockroach,mbertschler/cockroach,il9ue/cockroach,veteranlu/cockroach,cuongdo/cockroach,bg451/cockroach,cockroachdb/cockroach,glycerine/cockroach,nieyy/cockroach,bg451/cockroach,bg451/cockroach,arjun4084346/cockroach,mbertschler/cockroach,sunya123/cockroach,bg451/cockroach,glycerine/cockroach,vivekmenezes/cockroach,il9ue/cockroach,maxlang/cockroach,vaibhavarora/cockroach,vaibhavarora/cockroach,marcuswestin/cockroach,cockroachdb/cockroach,il9ue/cockroach,il9ue/cockroach,nieyy/cockroach,vaibhavarora/cockroach,cuongdo/cockroach,vaibhavarora/cockroach,nieyy/cockroach,sunya123/cockroach,cockroachdb/cockroach,cockroachdb/cockroach,sunya123/cockroach,cockroachdb/cockroach,mbertschler/cockroach,paperstreet/cockroach,marcuswestin/cockroach,maxlang/cockroach,vivekmenezes/cockroach,arjun4084346/cockroach,sunya123/cockroach,veteranlu/cockroach