Make the global #define IO_SIZE configurable for easier performance testing

Making mariadb's IO_SIZE configurable enables more straight-forward investigation of the performance implications of having an IO_SIZE which is different than the memory page size.

The default IO_SIZE of 4096 as defined in include/my_global.h and alsoin libmariadb's include/ma_global.h matches to the memory page size of most systems. However, Larger page sizes are widely supported, called "huge pages" in Linux, "superpages" in FreeBSD, and "large pages" in MS Windows.

On POSIX systems, obtaining the page size can be done via:

  page_size= sysconf(_SC_PAGESIZE);

On Windows:

  SYSTEM_INFO si;

  GetSystemInfo(&si);

  page_size= si.dwPageSize;

Note that libmariadb's include/ma_global.h should also be adjusted to avoid a double #define of IO_SIZE and to ensure they are defined to be the same.