abstract: reduced increased CPU consumption

[GRISHNOV]

Due to the fact that queue_state_fiber called box.ctrl.wait_r* every millisecond, the idle queue consumed more than 10% of CPU.

The proposed solution makes calls to box.ctrl.wait_r* blocking until the read/write mode is changed. Thus, CPU consumption is reduced to about ~1%.

Below is a performance comparison using a script t/benchmark/multi_consumer_work.lua whith params consumers-count = 650 and batch-size = 5000 on CPU AMD Ryzen 5 5600U.

For comparison, consider three commits. The first column is commit (9ff105b) before implementing queue_state_fiber. The second column is commit (159a43d) before implementation of the described proposal. That is, queue_state_fiber calls box.ctrl.wait_r* every millisecond. The third column (offered) is the current offer, the box.ctrl.wait_r* blocking call.

The data is given as an arithmetic mean of 25 measurement points:

[time\commit]	9ff105b	159a43d	offered
fill queue	8 197 452	8 567 415	8 751 545
task confirm	49 817 371	52 203 080	53 020 243

Final performance comparison as a percentage:

% down	9ff105b->159a43d	159a43d->offered	9ff105b->offered
fill queue	4.513% down	2.149% down	6.759% down
task confirm	4.788% down	1.565% down	6.429% down

As a result, the proposed solution causes a decrease in performance for fill queue 2.149% and task confirm 1.565%.

Closes #183