mod_proxy_fcgi: limit the flush buckets inserted when flushpackets=on|auto

This commit is a follow up of r1802040 based on Jacob's
feedback, namely inserting the FLUSH buckets only when
really needed and useful, not always.



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1807876 13f79535-47bb-0310-9956-ffa450edef68