Add TestSingleKey for stressing transactional operation upon a single key.
Add TestSingleKey for stressing transactional operation upon a single key.

For a 3 node cluster, running TestSingleKey results in ~1000 ops/min
which isn't that great. The max latency per operation is ~1sec. Also not
great, but its a starting point.

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ao0907/cockroach,maxlang/cockroach,jmank88/cockroach,mbonaci/cockroach,domluna/cockroach,ngaut/cockroach,loucash/cockroach,SustainFund/cockroach,abhishekgahlot/cockroach,domluna/cockroach,vaibhavarora/cockroach,hassyma/cockroach,XisiHuang/cockroach,maxlang/cockroach,mbonaci/cockroach,GokulSrinivas/cockroach,marcuswestin/cockroach,jgautheron/cockroach,abhishekgahlot/cockroach,loucash/cockroach,jmank88/cockroach,maxlang/cockroach,vivekmenezes/cockroach,marcuswestin/cockroach,briceburg/cockroach,cockroachdb/cockroach,nkhuyu/cockroach,freakynit/cockroach,arjun4084346/cockroach,freakynit/cockroach,glycerine/cockroach,maxlang/cockroach,c4pt0r/cockroach,bg451/cockroach,veteranlu/cockroach,chrisseto/cockroach,XisiHuang/cockroach,loucash/cockroach,linearb/cockroach,petermattis/cockroach,SustainFund/cockroach,jgautheron/cockroach,slavau/cockroach,chrisseto/cockroach,arypurnomoz/cockroach,slavau/cockroach,mbonaci/cockroach,ajayaa/cockroach