sql.py with_explain_comment(), DbConn: Fixed bug where with_explain_comment() was being run in per-row imports (row-based import and csv2db with INSERT), causing the overhead of an EXPLAIN query for every single INSERT and filling up the cache with EXPLAIN query results, by adding autoexplain mode, only running with_explain_comment() in autoexplain mode, and only enabling autoexplain mode for column-based import