sql.py: Added mk_select(), and use it in select()

bin/map: Always pass qual_name(table) to sql.select(). This is possible now that qual_name() can handle None schemas.

db_xml.py: put_table(): Take separate in_table and in_schema names, instead of in_table and table_is_esc, because the in_schema is needed to scope the temp tables appropriately

sql.py: qual_name(): If schema is None, don't prepend schema

bin/map, sql.py: Turned SQL query caching back on because benchmarks of just the caching on vs. off reveal that it does reduce processing time significantly. However, there is a slowdown that was introduced between the time caching was added and the time the same XML tree was used for each node, which was giving the false indication that the slowdown was due to the caching.

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