Overretainment of memory with many SELECT calshowpartitions() queries

Investigation following fix for MCOL-6064 brought to light memory retainment associated with SELECT text_returning_udf() queries. The problem shows up even when we effectively return a constant string, doing nothing in the body of a function.

We can look into server's code to find where retainment occurs, or we can create a information_schema.columnstore_partitions table for our users to select from it's rows and apply filters or joins.