[REGRESSION] Pool size is extended on each allocation

Reported in Boost mailing list: https://lists.boost.org/Archives/boost/2023/06/254750.php

951ca5725 changed malloc_need_resize() to use set_next_size(), which
sets start_size in addition to setting next_size. this causes repeated use
of purge_memory() to allocate 2x size after every use. an illustration:

```c++
#include
int main()
{
boost::pool pool(8);
for(int i = 0; i < 10; i++) {
printf("%d %ld\n", i, pool.get_next_size());
pool.purge_memory();
void *ptr = pool.malloc();
(void)ptr;
}
}
```

before the commit the output is:
```
0 32
1 64
2 64
3 64
4 64
5 64
6 64
7 64
8 64
9 64
```
after the commit, the output is:
```
0 32
1 64
2 128
3 256
4 512
5 1024
6 2048
7 4096
8 8192
9 16384
```