[v1.03] adds observers for cached and partially-cached responses

applies patch made to "moz-rewrite":
    https://github.com/warren-bank/moz-rewrite
in commit:
    https://github.com/warren-bank/moz-rewrite/commit/810924371d7034e2b1b79b99c1ed174548bf8441

https://developer.mozilla.org/en-US/docs/Observer_Notifications#HTTP_requests
  * 'http-on-examine-cached-response'
  * 'http-on-examine-merged-response'