dict_mem_table_create(): Account for DICT_TF_COMPRESSED in a debug assertion.

btr_store_big_rec_extern_fields(), btr_free_externally_stored_field(),
btr_copy_externally_stored_field(): Implement the disk format for
compressed BLOB pages.

btr_copy_externally_stored_field(): Improve error reporting and handling
when decompressing BLOB pages.

buf_flush_init_for_writing(), buf_page_is_corrupted(), buf_page_print():
Account for compressed BLOB pages (FIL_PAGE_TYPE_ZBLOB).

buf_calc_zblob_page_checksum(): New function.