Use a 32-bit data type for some tablespace fields

In the InnoDB data files, we allocate 32 bits for tablespace identifiers and page numbers as well as tablespace flags. But, in main memory data structures we allocate 32 or 64 bits, depending on the register width of the processor. Let us always use 32-bit fields to eliminate a mismatch and reduce the memory footprint on 64-bit systems.