sql.py: DbConn._db(): Record that a transaction is already open before setting the search_path so that a query is never run with an _savepoint value less than 1 (manual transactions are not supported yet)

sql.py: DbConn.with_savepoint(): Increment _savepoint before running queries so they don't get autocommitted

sql.py: empty_temp(): Empty temp tables even in debug_temp mode, so that it can be seen which tables have been garbage collected and disk space leaks can be detected. This will not affect the external re-runnability of slow queries in debug_temp mode, as long as the user aborts the debug_temp import while the slow query is still running.

sql_gen.py: ColDict: Use OrderedDict so that order of keys in input dict (if ordered) will be preserved. This should ensure that tempt table unique indexes have their columns in the same order as the output table, so that a merge join can be used.

util.py: dict_subset(): Use OrderedDict so that order of keys in input dict (if ordered) will be preserved

main Makefile: python-Darwin: Added pip installation instructions. python-Linux: Added ordereddict.

sql.py: DbConn.col_info(): cacheable param defaults to True now that callers explicitly turn off cacheable when needed

sql.py: add_index_col(): Explicitly set update()'s col_info caching depending on whether col_info will be changed later by add_not_null()

sql.py: mk_update(): Renamed cacheable param to cacheable_ so it wouldn't conflict with update()'s cacheable param

sql.py: mk_update(): Added cacheable param to set whether column structure information used to generate the query can be cached