"fast-start" the delivery of gzip-compressed bodies, and support flushing of GZIPOutputStreams on JDK7+

GZIPOutputStream.flush() is essentially a no-op under <= JDK6 given
http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=4813885. We weren't
calling it before anyway, but its JDK7 semantics are necessary when
compressing seq response bodies; otherwise, the middleware will require
complete consumption of the seq before a single byte hits the wire.
Without this change, gzipping of seq bodies that are truly lazy (e.g.
supporting server-sent events) is not practical; this patch therefore
also disables seq body support given pre-JDK7 GZIPOutputStream.flush()
functionality.