SAVE SNAPSHOT takes too much CPU

SAVE SNAPSHOT can be sped up quite a bit by avoiding costly iteration and doing I/O buffering more efficiently.
Currently it takes up to 80% CPU.

What needs to be done:
(done) - iter_inner and iter_outer removed
- kill conver_snap_row_to_wal
(done) - find the optimal value for setvbuf/IO cache
- experiment with allocating index memory with MAP_SHARED flag or quickly unmmapping it after fork().
- log_io error messages are often quite obscure. At least on Linux, we can extend them
with file names (using /proc filesystem):
static const char *
nfilename(int fd)
{
#ifdef TARGET_OS_LINUX
    char proc_path[32];
    static __thread char filename_path[PATH_MAX];

    sprintf(proc_path, "/proc/self/fd/%d", fd);

    ssize_t sz = readlink(proc_path, filename_path,
                  sizeof(filename_path));

    if (sz >= 0) {
        filename_path[sz] = '\0';
        return filename_path;
    }
#endif /* TARGET_OS_LINUX */
    return ""; /* Not implemented. */
}

- look into pipelining the write, when one thread is busy doing crc32 and formatting the data and another is writing to disk.

Each step ideally should be done separately.