Try to infer the time zone for timestamptzs from TimeZone

Firstly, keep track of the current value of TimeZone from the
ParameterStatus messages sent by the server.  Then, when receiving a
"timestamp with time zone" value from the database, try to set its
time zone based on the current value of TimeZone.  If, and only if,
the offset we got from the local time zone database matches the
offset we got from the server, we return the new value with time
zone information.  This way the user always gets the time as the
database sent it (even if the local database is not in sync with the
remote one), which maintains backwards compatibility and avoids
confusion.

This should currently work correctly in all cases, but if we ever
start supporting multiple simultaneous queries, we will have to be
careful to use the correct TimeZone value for each of them.