sql.py: DbConn: Added clear_cache() and reset() and use reset() in init()

bin/map: Use new DbConn.close()

sql.py: DbConn: Added close()

db_xml.py: partition_size: Set to just more than the size of the largest data source that was successfully imported in simultaneous import

db_xml.py: put_table(): Partition in_table if larger than a threshold. The threshold is initially set to disable partitioning. Partitioning will hopefully eliminate the excessive disk usage for large input tables, which has caused the system to run out of disk space due to what may be a bug in PostgreSQL.

db_xml.py: put_table(): Set in_table's default srcs to in_table itself instead of sql_gen.src_self, so that any copies of in_table will inherit the same srcs instead of being treated as a top-level table. This ensures that the top-level table's errors table will always be used.

sql_io.py: cast(): Always convert exceptions to warnings if the input is a column or expression, even if there is no place to save the errors, so that invalid data does not need to be handled by the caller in a (much slower) extra exception-handling loop

sql_io.py: put_table(): MissingCastException: When casting, handle InvalidValueException by filtering out invalid values with invalid2null() in a loop

sql_io.py: cast_temp_col(): Run sql.update() in recover mode in case expr produces errors. Don't cache sql.update() in case this function will be called again after error recovery.

sql.py: Generalized FunctionValueException to InvalidValueException so that it will match all invalid-value errors, not just those occurring in user-defined functions