Fix pool lifetime issues when the proxy backend connection is terminated

Fix pool lifetime issues when the proxy backend connection is terminated
early by forcing a setaside on transient buckets placed in the brigade
by mod_ssl. This has the effect of extending the lifetime of buckets until
the end of the request. This is a variation on the original fix for this
problem, which added transient buckets to be setaside later in the process.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1035504 13f79535-47bb-0310-9956-ffa450edef68