Avoid comparing sql.Types in dolt diff

[nicktobey]

Fixes #8511

Not all implementations of sql.Type are comparable. In particular, the enum type has a map field which makes the entire type incomparable.

To avoid doing this comparison in the chooseMostFlexibleType function, we modify it to compare the underlying type tag instead. This PR also short-circuits computing a unified schema unless the schema has changed.

[coffeegoddd]

@nicktobey DOLT

comparing_percentages
100.000000 to 100.000000

version	result	total
43196c0	ok	5937457

version	total_tests
43196c0	5937457

correctness_percentage
100.0

[fulghum]

LGTM!

[github-actions]

@coffeegoddd DOLT

test_name	detail	row_cnt	sorted	mysql_time	sql_mult	cli_mult
batching	LOAD DATA	10000	1	0.06	1.33
batching	batch sql	10000	1	0.07	1.57
batching	by line sql	10000	1	0.07	1.57
blob	1 blob	200000	1	0.88	3.76	3.53
blob	2 blobs	200000	1	0.9	4.09	4.29
blob	no blob	200000	1	0.86	2.4	2.06
col type	datetime	200000	1	0.81	2.91	2.81
col type	varchar	200000	1	0.67	3.3	2.84
config width	2 cols	200000	1	0.8	2.35	1.88
config width	32 cols	200000	1	1.84	1.98	2.2
config width	8 cols	200000	1	0.98	2.27	1.89
pk type	float	200000	1	0.85	2.22	1.82
pk type	int	200000	1	0.81	2.3	1.86
pk type	varchar	200000	1	1.48	1.66	1.38
row count	1.6mm	1600000	1	5.42	2.87	2.3
row count	400k	400000	1	1.39	2.74	2.19
row count	800k	800000	1	2.79	2.77	2.23
secondary index	four index	200000	1	3.42	1.39	1.08
secondary index	no secondary	200000	1	0.89	2.3	1.98
secondary index	one index	200000	1	1.1	2.34	1.95
secondary index	two index	200000	1	1.94	1.73	1.4
sorting	shuffled 1mm	1000000	0	5.16	2.68	2.27
sorting	sorted 1mm	1000000	1	5.02	2.74	2.34

[github-actions]

@coffeegoddd DOLT

name	detail	mean_mult
dolt_blame_basic	system table	1.11
dolt_blame_commit_filter	system table	2.96
dolt_commit_ancestors_commit_filter	system table	0.66
dolt_commits_commit_filter	system table	1
dolt_diff_log_join_from_commit	system table	2
dolt_diff_log_join_to_commit	system table	2.03
dolt_diff_table_from_commit_filter	system table	1.22
dolt_diff_table_to_commit_filter	system table	1.2
dolt_diffs_commit_filter	system table	0.97
dolt_history_commit_filter	system table	1.42
dolt_log_commit_filter	system table	1.1

[github-actions]

@coffeegoddd DOLT

name	add_cnt	delete_cnt	update_cnt	latency
adds_only	60000	0	0	0.73
adds_updates_deletes	60000	60000	60000	3.85
deletes_only	0	60000	0	1.89
updates_only	0	0	60000	2.43