137108: kv: allocate Batch{Request|Response} header and {requests|responses} together r=nvanbenschoten a=nvanbenschoten

These two commits avoid heap allocations in the common cases by allocating the `BatchRequest` and `BatchResponse` headers and their respective requests or responses together, in a single malloc.

Together, this avoids a pair of heap allocations per KV Batch operation.

```
name                                            old time/op    new time/op    delta
Sysbench/SQL/1node_remote/oltp_read_only-10       3.55ms ±17%    3.54ms ±16%    ~     (p=0.945 n=19+20)
Sysbench/SQL/1node_remote/oltp_write_only-10      2.13ms ±20%    2.13ms ±23%    ~     (p=0.531 n=19+20)
Sysbench/SQL/1node_remote/oltp_read_write-10      5.64ms ± 9%    5.77ms ±24%    ~     (p=0.988 n=18+19)
Sysbench/SQL/1node_remote/oltp_point_select-10     174µs ±10%     177µs ±19%    ~     (p=0.799 n=19+18)
Sysbench/KV/1node_remote/oltp_read_only-10         843µs ±17%     824µs ±15%    ~     (p=0.461 n=20+19)
Sysbench/KV/1node_remote/oltp_write_only-10        784µs ±14%     751µs ±24%    ~     (p=0.226 n=18+18)
Sysbench/KV/1node_remote/oltp_read_write-10       1.75ms ±15%    1.76ms ±21%    ~     (p=0.639 n=17+19)
Sysbench/KV/1node_remote/oltp_point_select-10     35.7µs ±16%    36.4µs ±25%    ~     (p=0.795 n=19+19)

name                                            old alloc/op   new alloc/op   delta
Sysbench/KV/1node_remote/oltp_point_select-10     6.57kB ± 0%    6.56kB ± 0%  -0.26%  (p=0.005 n=20+20)
Sysbench/SQL/1node_remote/oltp_read_only-10       1.15MB ± 1%    1.15MB ± 0%    ~     (p=0.191 n=19+19)
Sysbench/SQL/1node_remote/oltp_write_only-10       573kB ± 4%     573kB ± 4%    ~     (p=0.758 n=20+20)
Sysbench/SQL/1node_remote/oltp_read_write-10      1.64MB ± 1%    1.64MB ± 2%    ~     (p=0.771 n=19+20)
Sysbench/SQL/1node_remote/oltp_point_select-10    30.0kB ± 1%    30.1kB ± 1%    ~     (p=0.632 n=20+19)
Sysbench/KV/1node_remote/oltp_read_only-10         656kB ± 0%     655kB ± 0%    ~     (p=0.067 n=18+20)
Sysbench/KV/1node_remote/oltp_write_only-10        263kB ± 2%     263kB ± 2%    ~     (p=0.961 n=19+16)
Sysbench/KV/1node_remote/oltp_read_write-10        879kB ± 1%     880kB ± 1%    ~     (p=0.495 n=20+20)

name                                            old allocs/op  new allocs/op  delta
Sysbench/KV/1node_remote/oltp_point_select-10       61.0 ± 0%      59.0 ± 0%  -3.28%  (p=0.000 n=20+20)
Sysbench/KV/1node_remote/oltp_read_only-10         1.87k ± 0%     1.84k ± 0%  -1.50%  (p=0.000 n=17+17)
Sysbench/KV/1node_remote/oltp_read_write-10        3.46k ± 0%     3.42k ± 0%  -1.38%  (p=0.000 n=20+19)
Sysbench/KV/1node_remote/oltp_write_only-10        1.59k ± 0%     1.57k ± 0%  -1.32%  (p=0.000 n=20+18)
Sysbench/SQL/1node_remote/oltp_write_only-10       3.98k ± 1%     3.94k ± 1%  -1.00%  (p=0.000 n=20+20)
Sysbench/SQL/1node_remote/oltp_read_only-10        4.78k ± 1%     4.74k ± 1%  -0.76%  (p=0.000 n=19+19)
Sysbench/SQL/1node_remote/oltp_point_select-10       266 ± 1%       264 ± 1%  -0.64%  (p=0.003 n=20+19)
Sysbench/SQL/1node_remote/oltp_read_write-10       8.74k ± 0%     8.68k ± 1%  -0.60%  (p=0.000 n=19+18)
```

Epic: None
Release note: None

Co-authored-by: Nathan VanBenschoten <[email protected]>