Refactor the input buffer filter to use a context as the output buffer

filter does. Ensure that the filter properly handles the non blocking
case, both when a downstream filter returns EAGAIN, or if a filter
returns APR_SUCCESS and an empty brigade.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@718082 13f79535-47bb-0310-9956-ffa450edef68