Make warning limit per-query instead of per-connection

[royi-luo]

Currently, the limit on the number of cached warnings (configurable using CALL warning_limit={value}) is applied per-connection. We should discuss if it is more user-intuitive for this limit to apply per-query instead, and whether there is a need for a global limit on the number of cached warnings.

For reference, duckDB's rejects_limit setting for read_csv() applies for each input file (which is even more granular than per-query).