sql.py: DbConn: Added config option debug_temp

sql.py: function_exists(): Fixed bug where trigger functions needed to be excluded, since they cannot be called directly

sql.py: Added function_exists()

sql_gen.py: Made Function an alias of Table so that isinstance(..., Function) will always work correctly

sql_gen.py: Added as_Function()

sql.py: put_table(): Lock the output table in EXCLUSIVE mode before getting its pkey so that an ACCESS SHARE lock is not acquired before EXCLUSIVE (causing a lock upgrade and deadlock). This race condition may not have been previously noticeable because pkey() is cached, so calling it doesn't necessarily execute a query or acquire an ACCESS SHARE lock.

sql.py: put_table(): Document that must be run at the beginning of a transaction

sql.py: put_table(), mk_select(): Switched back to having put_table() acquire the EXCLUSIVE locks, but right at the beginning of the transaction, in order to avoid lock upgrades which cause deadlocks

sql.py: with_autocommit(): Only allow turning autocommit on, because the opposite is not meaningful and may conflict with the session-global isolation level

sql.py: DbConn: Set the transaction isolation level to READ COMMITTED using set_isolation_level() so that the isolation level affects all transactions in the session, not just the current one