Fix equals-method of the wrapper returned by PGXAConnection.getConnection()

This is fixes the long-standing issue discussed here:

http://www.postgresql.org/message-id/F3EFBAE2-4B9A-4D5A-B436-02BF1C0A0DA0@leonisapps.pl

This is the patch by Florent Guillaume from http://www.postgresql.org/message-id/CAF-4BpPdTUVnZQd-Y70-uSHVUJ-hfxLmfCW+MBL1i1AEaLoPMg@mail.gmail.com. I just added a few comments