For compatibility with diagnostic software, let us
return a dummy buffer pool identifier 0 and restore
the columns that were initially deleted in
commit 1a6f708e:

	information_schema.innodb_buffer_page.pool_id
	information_schema.innodb_buffer_page_lru.pool_id
	information_schema.innodb_buffer_pool_stats.pool_id
	information_schema.innodb_cmpmem.buffer_pool_instance
	information_schema.innodb_cmpmem_reset.buffer_pool_instance

Thanks to Vladislav Vaintroub for pointing this out.