Added optional support for caching streamed responses in mod_cache.

Added optional support for caching streamed responses in mod_cache.
Notes:
  * I've created a new config directive CacheMaxStreamingBuffer,
    to set the maximum amount of data that mod_cache will buffer
    per request if it hasn't yet seen an EOS.  The default is
    zero, which preserves the original behavior: cache only if
    the response has a known content-length or all the content
    is available in the first brigade passed to the CACHE_IN filter.
  * A big block of code in cache_in_filter() got wrapped in an
    if-statement in this change.  To make the diff more readable,
    I'm committing without indentation changes; a second commit
    will include (only) the indentation update.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@96631 13f79535-47bb-0310-9956-ffa450edef68