Made the mc_transaction_t pulse the disk_ack_signal in its destructor, sometimes fakely.
Made ~mc_transaction_t handle the disk_ack_signal == NULL case.
Made mc_transaction_t use its own wait_for_flush field (albeit taken from the const field in the writeback).
Removed mc_transaction_t::wait_for_flush, just used null/non-nullness of disk_ack_signal.
Removed the defunct wait_for_flush variable and config parameter.
Made disk_ack_signals be a sync_callback_t.
Made sync_callback be passed directly to the writeback instead of holding up transaction destruction.
Made btree_slice_t::create wait for its disk_ack_signal.
Made a read-only mc_transaction_t constructor, did not use it anywhere.