mod_cache: Invalidate cached entities in response to RFC2616 Section

13.10 Invalidation After Updates or Deletions. PR 15868

trunk patch: http://svn.apache.org/r1070179
             http://svn.apache.org/r1478140
             http://svn.apache.org/r1478173
             http://svn.apache.org/r1478798
2.4.x patch: http://people.apache.org/~minfrin/httpd-mod_cache-invalidate7.patch

Submitted by: minfrin
Reviewed by: jim, wrowe


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1487102 13f79535-47bb-0310-9956-ffa450edef68