fix case insensitive column match in kv iter

[jycor]

When building the keyLookupMapper for the special prolly tree kvIter, we match column names by exact casing. This leads to incorrect lookup indexes.

The fix was to use lowercase when matching.

companion pr: dolthub/go-mysql-server#2640

[max-hoffman]

lgtm, maybe move nil check inside valid?

[coffeegoddd]

@jycor DOLT

comparing_percentages
100.000000 to 100.000000

version	result	total
e77920b	ok	5937457

version	total_tests
e77920b	5937457

correctness_percentage
100.0

[coffeegoddd]

@jycor DOLT

comparing_percentages
100.000000 to 100.000000

version	result	total
31083ab	ok	5937457

version	total_tests
31083ab	5937457

correctness_percentage
100.0

[coffeegoddd]

@jycor DOLT

comparing_percentages
100.000000 to 100.000000

version	result	total
dcfce45	ok	5937457

version	total_tests
dcfce45	5937457

correctness_percentage
100.0