bin/map: Turn SQL query caching off by default

bin/map: Added cache_sql env var to enable SQL query caching

sql.py: Make caching DbConn enablable. Turn caching off by default because recent benchmarks (n=1000) were showing that it slows things down.

bin/map: Added new verbose_errors mode, enabled in test mode and off otherwise, which controls whether the output row and tracebacks are included in error messages. Having this off in import mode will reduce the size of error logs so they don't fill up the vegbiendev hard disk as quickly.

exc.py: print_ex(): Added detail option to turn off traceback

bin/map: Turn parallel processing off by default. This should fix "Cannot allocate memory" errors in large imports.

bin/map: in_is_db: Don't cache the main SELECT query

bin/map: by_col: Use the created template, which already has the column names in it, instead of mapping a sample row

bin/map: Fixed bug where db_xml could not be imported twice, or it was treated as an undefined variable for some reason

bin/map: map_table(): Make each column a db_xml.ColRef instead of a bare index, so that it will appear as the column name when converted to a string. This will provide better debugging info in the template tree and also avoid needing to create a separate sample row in by_col.