Backport the body pre-fetch code from trunk/, modifying the three
  stream_te / stream_cl / spool_cl functions to presume an input_brigade
  before fighting with fetching additional body content.

  We will be using the bytes_read in a later commit to make a better
  decision about choosing to send a C-L or chunked body to the backend.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/proxy-reqbody-2.0.x@230724 13f79535-47bb-0310-9956-ffa450edef68