Get rid of O(n^2) scan in dyn array (mtr->memo) operations, accessing
the dyn array blocks directly.

dyn_array_get_last_block(), dyn_array_get_next_block(),
dyn_array_get_prev_block(): Define as a constness-preserving macro.

Add const qualifiers to many dyn_array functions.

mtr_memo_slot_release_func(): Renamed from mtr_memo_slot_release():
Make mtr_t* a debug-only parameter. Assume that slot->object != NULL.

mtr_memo_pop_all(): Access the dyn_array blocks directly, replacing
O(n^2) operation with O(n).

mtr_memo_release(): Access the dyn_array blocks directly, replacing
O(n^2) operation with O(n). This caused the performance problem.

rb#1540 approved by Jimmy Yang