Add performance tests to compare crud and vshard

[DifferentialOrange]

This patch adds new cases for performance tests: select for equal
conditions for primary and secondary indexes (including known bucket_id
case from #220), and adds corresponding vshard test cases to compare
performance. Comparison may be not exactly precise since vshard test
functions use naive mergers, but should at least estimate basic
differences.

Test run on HP ProBook 440 G7 i7/16Gb/256SSD shows that CRUD module is
3-6 times slower than vshard calls for prepared functions for select and
1.6 times slower for insert.

HP ProBook 440 G7 i7/16Gb/256SSD

SUCCESS REQUESTS PER SECOND

(The higher the better)

	vshard	crud (stats disabled)	crud (stats disabled, known bucket_id)
select by pk	81824.72	23936.52	23992.12
select gt by pk (limit 10)	15201.35	4305.91
select eq by secondary (limit 10)	14783.89	4835.79
select eq by sharding secondary	79791.30	13689.91	18376.29
insert	84113.60	51847.18

AVERAGE CALL TIME

(The lower the better)

	vshard	crud (stats disabled)	crud (stats disabled, known bucket_id)
select by pk	2.444 ms	8.355 ms	8.335 ms
select gt by pk (limit 10)	13.156 ms	46.446 ms
select eq by secondary (limit 10)	13.528 ms	41.357 ms
select eq by sharding secondary	2.506 ms	14.604 ms	10.881 ms
insert	7.132 ms	11.571 ms

MAX CALL TIME

(The lower the better)

	vshard	crud (stats disabled)	crud (stats disabled, known bucket_id)
select by pk	17.277 ms	37.793 ms	36.457 ms
select gt by pk (limit 10)	41.015 ms	103.195 ms
select eq by secondary (limit 10)	43.169 ms	98.459 ms
select eq by sharding secondary	15.934 ms	71.610 ms	58.771 ms
insert	66.039 ms	72.328 ms

CI Runner

See https://github.com/tarantool/crud/runs/5421561722?check_suite_focus=true

Several minor fixes also was introduced.

Fix replicaset alias in test cluster. Disable checks for perf tests.
Return tuples from pairs perf case.

Fix running separate tests. After this patch, developer can run luatest
with -p flag for perf tests to get results only for requested tests.
Example:

PERF_MODE_ON=true ./.rocks/bin/luatest -c -p perf.test_crud_pairs_gt_with_stats_disabled

• Tests
• Changelog (I don't think it should bother users)
• Documentation (There's nothing to document)

Closes #225

[DifferentialOrange]

PR is ready for review, but since it depends on #244 improvements, I will not request any reviews for now (until #244 is merged).

[Totktonada]

LGTM.

Quite interesting results, thanks!

I left some minor comments. Feel free to fix or go ahead.