Merge branch 'reuse-conn_address-on-retry-v11'

There is regression tests and also this code worked/tested during crawling a
huge number of pages (billions).

* reuse-conn_address-on-retry-v11:
  be_sock: bufferevent_socket_set_conn_address(): assert instead of silent no-op
  http: reuse connected address only with EVHTTP_CON_REUSE_CONNECTED_ADDR
  be_sock: sanity check in bufferevent_socket_set_conn_address()
  be: replace sockaddr_storage with sockaddr_in6 for conn_address
  be: we don't need to use getpeername() we we have conn_address
  be: replace conn_address by full struct instead of pointer
  test/http: cover retrying with saved conn_address by shutting down dns server
  http: use IP address that we got before (if any) during retrying
  bufferevent: move conn_address out from http into bufferevent
  be: make @sa const for bufferevent_socket_connect()
  util: make @sa const for evutil_socket_connect_()