storage: avoid seeking iterators already at target
storage: avoid seeking iterators already at target

```
name                              old time/op    new time/op    delta
MVCCConditionalPutCreate10-8        5.35µs ± 9%    5.13µs ± 2%  -4.04%        (p=0.002 n=10+10)
MVCCConditionalPutCreate100-8       5.34µs ± 1%    5.28µs ± 1%  -1.13%        (p=0.011 n=10+10)
MVCCConditionalPutCreate1000-8      7.22µs ± 2%    7.20µs ± 2%    ~             (p=0.546 n=9+9)
MVCCConditionalPutCreate10000-8     30.5µs ± 5%    28.9µs ± 1%  -5.04%        (p=0.001 n=10+10)
MVCCConditionalPutReplace10-8       7.01µs ± 3%    6.87µs ± 1%    ~            (p=0.053 n=9+10)
MVCCConditionalPutReplace100-8      7.71µs ± 7%    7.14µs ± 2%  -7.49%        (p=0.000 n=10+10)
MVCCConditionalPutReplace1000-8     14.1µs ± 8%    12.9µs ± 0%  -8.73%         (p=0.000 n=10+8)
MVCCConditionalPutReplace10000-8    56.0µs ± 5%    53.8µs ± 1%  -3.85%        (p=0.005 n=10+10)
```

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