[4pt] Investigate merger/key_def embedding into current implementation

[olegrok]

Write test branch for 2.4+ Tarantool versions until tarantool/tarantool#5273 and tarantool/tarantool#5384 are not implemented

[olegrok]

See https://github.com/Totktonada/tarantool-merger-examples/tree/master/chunked_example_fast

[Totktonada]

I would be interested in performance testing results.

A measurement may be more close to a real life if we would imitate a delay between a router and a storage:

tc qdisc add dev lo root handle 1:0 netem delay 500msec  # 1ms rtt
tc qdisc del dev lo root  # clean up

However microbenchmark is interesting too.

[olegrok]

I have simple benchmark - https://github.com/tarantool/crud/tree/perf-test

But at first glance I see some slowdown after merger integration (introduced here #38).
Seems we need to cache key_def instances (Currently this optimization is switched off). Currently we have implementation that passes our tests but further investigation is required to estimate and solve performance problems.