Add a flush bucket type. This bucket advises filters to flush any data

Add a flush bucket type.  This bucket advises filters to flush any data
they are currently storing.  There is no way we can force them to flush,
but we can advise.  This also adds the code to ap_rflush to use flush
buckets, although it isn't enabled yet.  I will enable it once we remove
buff from the code.  I also removed all calls to ap_rflush that are either
immediately before or immediately after a call to ap_finalize_protocol.
ap_finalize_protocol sends an EOS bucket, which also advises filters to
flush their data, so having both calls right next to each other is
redundant.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@86619 13f79535-47bb-0310-9956-ffa450edef68