divide _evhttp_ module into _evhttp_ and _evhttpconn_
update `ev_httpconn_init()` no longer accepts buffer size (redundant)
Add controlling interface for quitting

- accepts controlling command via a datagram socket.
- add "graceful quit" controlling command
- add "get the number of connections" controll command
- fix memory leaks by not calling `hdrstore_free()` in `ev_httpconn_stop()`
- fix memory leaks in `hdrstore_free()`
- fix a segmentation fault when `hdrstore_dump()` is called
- add `tcp_open()` to call socket, bind().
- remove `tcp_listen()` and similar functions.
multi thread support

Unfortunately, this degrade the performance.
idle loop starts iff quitting signal is received

- fix a bug of dealing Connection header
- fix a bug that was not working properly when EOF is met.
repeating `accept()` until EAGAIN is met
add new *ev_httpconn* callback for r/w

This may sacrifice the latency in favor of increasing TPS.
Before this commit, *ev_httpconn* uses `ev_httpconn_read_cb` and `ev_httpconn_write_cb`.  Switching both callback requires the control to transfer from one of the callbacks to *libev*.

Now, *ev_httpconn* uses `ev_httpconn_io_cb`, which does not transfer the control to *libev* to switching read/write mode.  This may speed up the handling multiple requests in the same connection (a.k.a keepalive), but since it does not release the control flow, so that requests from other connections may not have a chance to be served.