This patch merges the identical methods from the JDBC1 and JDBC2

This patch merges the identical methods from the JDBC1 and JDBC2
connection implementations (org.postgresql.jdbc[1|2].Connection) into
their superclass (org.postgresql.Connection).

It also changes the close() methods of Connection and PG_Stream, so that
PG_Stream no longer is responsible for sending the termination packet 'X'
to the backend. I figured that protocol-level stuff like that belonged in
Connection more than in PG_Stream.

Anders Bengtsson