buf_page_t: Remove the buf_pool pointer. Add buf_pool_index:6 next to
buf_fix_count:19 (it was buf_fix_count:25). This will limit the number
of concurrent transactions to somewhere around 524,288.

buf_pool_index(buf_pool): A new function to determine the index of a
buffer pool in buf_pool_ptr[].

buf_pool_ptr: Make this a dynamically allocated array instead of an
array of pointers. Allocate the array in buf_pool_init() and free in
buf_pool_free().

buf_pool_free_instance(): No longer free the buf_pool object, as it is
allocated from a big array.

buf_pool_from_bpage(), buf_pool_from_block(): Move the definitions to
the beginning of the files, because some compilers have (had) problems
with forward definitions of inline functions. Calculate the buffer
pool from buf_pool_index.

buf_pool_from_array(): Add debug assertions for input validation.