stroage: MVCCDeleteRange with iterator
stroage: MVCCDeleteRange with iterator

Fixes #4489.

```
name                               old time/op    new time/op    delta
MVCCDeleteRange1Version8Bytes-8       122ms ±10%      93ms ± 1%  -24.02%  (p=0.000 n=10+10)
MVCCDeleteRange1Version32Bytes-8     92.2ms ± 4%    84.9ms ± 6%   -7.95%  (p=0.000 n=10+10)
MVCCDeleteRange1Version256Bytes-8    25.3ms ± 4%    23.0ms ± 3%   -9.10%  (p=0.000 n=10+10)

name                               old speed      new speed      delta
MVCCDeleteRange1Version8Bytes-8    4.30MB/s ±11%  5.64MB/s ± 1%  +31.11%  (p=0.000 n=10+10)
MVCCDeleteRange1Version32Bytes-8   5.69MB/s ± 5%  6.18MB/s ± 7%   +8.66%  (p=0.000 n=10+10)
MVCCDeleteRange1Version256Bytes-8  20.7MB/s ± 4%  22.8MB/s ± 3%   +9.99%  (p=0.000 n=10+10)

name                               old alloc/op   new alloc/op   delta
MVCCDeleteRange1Version8Bytes-8      4.15MB ± 0%    0.30MB ± 0%  -92.76%    (p=0.000 n=9+8)
MVCCDeleteRange1Version32Bytes-8     2.81MB ± 0%    0.42MB ± 0%  -85.03%  (p=0.000 n=10+10)
MVCCDeleteRange1Version256Bytes-8    1.10MB ± 0%    0.53MB ± 0%  -52.04%   (p=0.000 n=10+7)

name                               old allocs/op  new allocs/op  delta
MVCCDeleteRange1Version8Bytes-8       37.5k ± 0%     28.1k ± 0%  -25.12%  (p=0.000 n=10+10)
MVCCDeleteRange1Version32Bytes-8      26.3k ± 0%     19.7k ± 0%  -25.13%   (p=0.000 n=10+9)
MVCCDeleteRange1Version256Bytes-8     6.93k ± 0%     5.18k ± 0%  -25.17%   (p=0.000 n=8+10)
```

And in SQL:
```
name                   old time/op    new time/op    delta
Delete1_Cockroach-8       787µs ± 4%     773µs ± 5%    ~     (p=0.143 n=10+10)
Delete10_Cockroach-8     2.47ms ± 2%    2.46ms ± 3%    ~      (p=0.780 n=9+10)
Delete100_Cockroach-8    17.5ms ± 3%    16.8ms ± 2%  -4.22%   (p=0.000 n=9+10)

name                   old alloc/op   new alloc/op   delta
Delete1_Cockroach-8      35.3kB ± 0%    34.7kB ± 0%  -1.81%  (p=0.000 n=10+10)
Delete10_Cockroach-8      141kB ± 0%     135kB ± 0%  -4.52%  (p=0.000 n=10+10)
Delete100_Cockroach-8    1.15MB ± 0%    1.09MB ± 0%  -5.60%    (p=0.000 n=8+9)

name                   old allocs/op  new allocs/op  delta
Delete1_Cockroach-8         586 ± 0%       580 ± 0%  -1.02%  (p=0.000 n=10+10)
Delete10_Cockroach-8      1.82k ± 0%     1.76k ± 0%  -3.30%  (p=0.000 n=10+10)
Delete100_Cockroach-8     13.5k ± 0%     12.9k ± 0%  -4.45%   (p=0.000 n=8+10)
```

I'll expect bigger % gains on SQL side if/when we make simple deletes skip scans.

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