sql: make the KVScan.*Native benchmarks use a transaction
sql: make the KVScan.*Native benchmarks use a transaction

This provides a more apples-to-apples comparison with the KVScan.*SQL
benchmarks. The numbers indicate there is room for optimization in small
read-only transactions.

name                   old time/op  new time/op   delta
KVScan1_Native-32      61.8µs ± 3%   85.9µs ± 2%  +38.98%   (p=0.000 n=9+10)
KVScan10_Native-32     95.3µs ± 5%  113.1µs ± 4%  +18.67%  (p=0.000 n=10+10)
KVScan100_Native-32     338µs ± 3%    355µs ± 5%   +5.09%  (p=0.000 n=10+10)
KVScan1000_Native-32   2.65ms ± 2%   2.67ms ± 4%     ~      (p=0.400 n=9+10)
KVScan10000_Native-32  29.8ms ± 3%   30.1ms ± 4%     ~      (p=0.400 n=10+9)

glycerine/cockroach,sunya123/cockroach,paperstreet/cockroach,maxlang/cockroach,sunya123/cockroach,sunya123/cockroach,vivekmenezes/cockroach,veteranlu/cockroach,sunya123/cockroach,petermattis/cockroach,cockroachdb/cockroach,petermattis/cockroach,cockroachdb/cockroach,sunya123/cockroach,sunya123/cockroach,paperstreet/cockroach,arjun4084346/cockroach,maxlang/cockroach,petermattis/cockroach,arjun4084346/cockroach,petermattis/cockroach,petermattis/cockroach,vaibhavarora/cockroach,glycerine/cockroach,arjun4084346/cockroach,maxlang/cockroach,arjun4084346/cockroach,veteranlu/cockroach,vaibhavarora/cockroach,vaibhavarora/cockroach,glycerine/cockroach,arjun4084346/cockroach,cockroachdb/cockroach,vivekmenezes/cockroach,maxlang/cockroach,vivekmenezes/cockroach,vivekmenezes/cockroach,vaibhavarora/cockroach,vaibhavarora/cockroach,sunya123/cockroach,maxlang/cockroach,vaibhavarora/cockroach,veteranlu/cockroach,veteranlu/cockroach,paperstreet/cockroach,vivekmenezes/cockroach,glycerine/cockroach,glycerine/cockroach,maxlang/cockroach,sunya123/cockroach,veteranlu/cockroach,arjun4084346/cockroach,paperstreet/cockroach,paperstreet/cockroach,arjun4084346/cockroach,maxlang/cockroach,glycerine/cockroach,vivekmenezes/cockroach,maxlang/cockroach,paperstreet/cockroach,petermattis/cockroach,cockroachdb/cockroach,paperstreet/cockroach,glycerine/cockroach,petermattis/cockroach,glycerine/cockroach,arjun4084346/cockroach,veteranlu/cockroach,paperstreet/cockroach,cockroachdb/cockroach,veteranlu/cockroach,vaibhavarora/cockroach,vivekmenezes/cockroach,cockroachdb/cockroach,cockroachdb/cockroach,vaibhavarora/cockroach,petermattis/cockroach,vivekmenezes/cockroach,veteranlu/cockroach,cockroachdb/cockroach