InnoDB keeps track of buffer-fixed buf_block_t or acquired rw_lock_t
within a mini-transaction. There are some memo_contains assertions
in the code that document when certain blocks or rw_locks must be held.
But, these assertions only check the mini-transaction memo, not the fact
whether the rw_lock_t are actually being held by the caller.

btr_pcur_store_position(): Remove #ifdef, and assert that the block
is always buffer-fixed.

rtr_pcur_getnext_from_path(), rtr_pcur_open_low(),
ibuf_rec_get_page_no_func(), ibuf_rec_get_space_func(),
ibuf_rec_get_info_func(), ibuf_rec_get_op_type_func(),
ibuf_build_entry_from_ibuf_rec_func(), ibuf_rec_get_volume_func(),
ibuf_get_merge_page_nos_func(), ibuf_get_volume_buffered_count_func()
ibuf_get_entry_counter_low_func(), page_set_ssn_id(),
row_vers_old_has_index_entry(), row_vers_build_for_consistent_read(),
row_vers_build_for_semi_consistent_read(),
trx_undo_prev_version_build():
Make use of mtr_memo_contains_page_flagged().

mtr_t::memo_contains(): Take a const memo. Assert rw_lock_own().

FindPage, FlaggedCheck: Assert rw_lock_own_flagged().