storage/engine: optimize iteration over inline values
storage/engine: optimize iteration over inline values

When iterating over inline values, we can use iter.Next() to get to the
next entry as opposed to the more general, but slower, iter.Seek().

name                old time/op    new time/op    delta
ReplicaSnapshot-32     207ms ± 7%     151ms ± 2%  -26.80%  (p=0.000 n=20+18)

name                old alloc/op   new alloc/op   delta
ReplicaSnapshot-32     162MB ± 0%     162MB ± 0%   -0.01%  (p=0.022 n=20+18)

name                old allocs/op  new allocs/op  delta
ReplicaSnapshot-32      117k ± 0%      117k ± 0%   -0.06%  (p=0.000 n=20+20)

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