Prevent temporary table name collisions

[angelamayxie]

fixes #10353

MySQL allows creating temporary tables with the same names as existing non-temporary tables and creating non-temporary tables with the same names as existing temporary tables. However, temporary tables cannot have the same name as other temporary tables.

Our previous implementation seemed to have assumed temporary tables with the same names were allowed. Each session had a map of db names to an array of tables, and new temporary tables were added to the end of the array, without checking if there was a table with an existing name. When fetching or dropping a temporary table, we iterated through the array and returned/dropped the first table with a matching name; this meant even though we allowed temporary tables with the same name, we only ever operated on whichever one was created first.

Since temporary tables with the same names are actually not allowed, the array of temporary tables was replaced with a name-to-table map to make fetching a temporary table with GetTemporaryTable faster. This also makes DropTemporaryTable faster. This does make GetAllTemporaryTables slower since we now have to iterate over the mappings to create an array for temporary tables, but GetAllTemporaryTables doesn't seem to be called as frequently as GetTemporaryTable.

[coffeegoddd]

@angelamayxie DOLT

comparing_percentages
100.000000 to 100.000000

version	result	total
1af6306	ok	5937471

version	total_tests
1af6306	5937471

correctness_percentage
100.0

[coffeegoddd]

@angelamayxie DOLT

comparing_percentages
100.000000 to 100.000000

version	result	total
06b3991	ok	5937471

version	total_tests
06b3991	5937471

correctness_percentage
100.0

[jycor]

LGTM