Raise NotSupportedError on PostgreSQL versions without two-phase commit
Handle missing isolation levels on ancient PostgreSQL versions
Return None instead of cffi NULL on missing parameter status value

Also don't leak ffi values from _get_equote
Protect _pq_execute against concurrent execution on the same connection
Typecasters must be able to receive a None

It may not happen in regular query execution, but it might with typecasters
calling other typecasters.
Some optimizations in the reading loop
Make sure the NumberRangeAdapter returns str, not unicode

Patch applied upstream.