Merging JDBC1 and JDBC2 similarities

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
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A n d e r s B e n g t s s o n ndrsbngtssn(at)yahoo(dot)se
Stockholm, Sweden