[no-release-notes] follow up fixes to mcv calculation

[max-hoffman]

Previous bug was only saving last MCV. Fix and add better testing.

[max-hoffman]

benchmark

[github-actions[bot]]

@max-hoffman workflow run: https://github.com/dolthub/dolt/actions/runs/9620880916

[coffeegoddd]

@max-hoffman DOLT

comparing_percentages
100.000000 to 100.000000

version	result	total
53d0215	ok	5937457

version	total_tests
53d0215	5937457

correctness_percentage
100.0

[coffeegoddd]

@max-hoffman DOLT	test_name	from_latency_p95	to_latency_p95	is_faster
tpcc-scale-factor-1	73.13	74.46	0

test_name	server_name	server_version	tps	test_name	server_name	server_version	tps	is_faster
tpcc-scale-factor-1	dolt	12f2a8d540a17172f3c8c8f0d88e3e82cb7b7aec	33.18	tpcc-scale-factor-1	dolt	53d0215ccfa1c407c780908cbd0309d957de9648	32.53	0

[coffeegoddd]

@max-hoffman DOLT	read_tests	from_latency_median	to_latency_median	is_faster
covering_index_scan	2.91	2.91	0
groupby_scan	17.63	17.63	0
index_join	5.28	5.28	0
index_join_scan	2.22	2.22	0
index_scan	53.85	53.85	0
oltp_point_select	0.52	0.52	0
oltp_read_only	8.28	8.28	0
select_random_points	0.81	0.83	0
select_random_ranges	0.97	0.97	0
table_scan	55.82	55.82	0
types_table_scan	142.39	142.39	0

write_tests	from_latency_median	to_latency_median	is_faster
oltp_delete_insert	6.21	6.21	0
oltp_insert	3.02	3.02	0
oltp_read_write	15.0	15.27	0
oltp_update_index	3.13	3.19	0
oltp_update_non_index	3.07	3.07	0
oltp_write_only	6.55	6.67	0
types_delete_insert	6.79	6.91	0