Fix for ticket #3020: set MySQL session time_zone variable to UTC ('+0:00') so TIMEST...

Fix for ticket #3020: set MySQL session time_zone variable to UTC ('+0:00') so TIMESTAMP column values are comparable against our other UTC timestamp values.

MySQL stores TIMESTAMP columns as UTC, but with a local time interface. (SRSLY?!) DATETIME columns are always bare and assumed to be local time, but we keep only UTC in them.
Forcing the session time_zone to UTC means we won't have to worry as much about what we're sending/receiving in there.

Also will let us remove the hack in master commit a7abb2323e7d57125b9fbc903a1cecc06c27944e for session tweaks