http: reset connection before installing retry timer (fix http retries handling)

This will fix some invalid read/write:
```
==556== Invalid read of size 8
==556==    at 0x4E4EEC6: event_queue_remove_timeout (minheap-internal.h:178)
==556==    by 0x4E508AA: event_del_nolock_ (event.c:2764)
==556==    by 0x4E53535: event_base_loop (event.c:3088)
==556==    by 0x406FCFA: dispatch (libcrawl.c:271)
==556==    by 0x402863: main (crawler.c:49)
==556==  Address 0x68a3f18 is 152 bytes inside a block of size 400 free'd
==556==    at 0x4C29C97: free (in /usr/local/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==556==    by 0x406F140: renew (libcrawl.c:625)
==556==    by 0x4E6CDE9: evhttp_connection_cb_cleanup (http.c:1331)
==556==    by 0x4E6E2B2: evhttp_connection_cb (http.c:1424)
==556==    by 0x4E4DF2D: bufferevent_writecb (bufferevent_sock.c:310)
==556==    by 0x4E52D1D: event_process_active_single_queue (event.c:1584)
==556==    by 0x4E53676: event_base_loop (event.c:1676)
==556==    by 0x406FCFA: dispatch (libcrawl.c:271)
==556==    by 0x402863: main (crawler.c:49)
But this one because of some invalid write before (I guess).
```

It is 100% reproduced during massive crawling (because this process
has many different servers), but after spending some time for trying to
reproduce this using some simple tests/utils I gave up for a few days (I
have a lot of work to do), but I'm sending this patch as a reminder.

Just in case, I've tried next tests:
- mixing timeouts/retries
- shutdown http server and return it back
- slow dns server for first request
- sleep before accept
- hacking libevent sources to change the behaviour of http layer (so it
  will go into that function which I'm insterested in).

Any notes/suggestions are welcome!