db_xml.py: put_table(): Partitioning in_table: Adjust bounds of last partition to actual row #s included

sql.py: DbConn: Added _ to reset() to indicate that it's a protected method and users should not call it directly

sql.py: DbConn.close(): Reset the connection completely using reset()

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