Set libpq sslcompression to off by default

 Set libpq sslcompression to off by default
 
 Since SSL compression is no longer recommended, turn the default in
 libpq from on to off.
 
 OpenSSL 1.1.0 and many distribution packages already turn compression
 off by default, so such a server won't accept compression anyway.  So
 this will mainly affect users of older OpenSSL installations.
 
 Also update the documentation to make clear that this setting is no
 longer recommended.
 
 Discussion: https://www.postgresql.org/message-id/flat/595cf3b1-4ffe-7f05-6f72-f72b7afa7993%402ndquadrant.com