do not fail while sending on http connections the client closed.

do not fail while sending on http connections the client closed.

when sending chunked requests via multiple calls to evhttp_send_reply_chunk,
the client may close the connection before the server is done sending. this
used to cause a crash.

we introduce a new function evhttp_request_get_connection() that allows the
server to determine if the request is still associated with a connection.
If it's not, evhttp_request_free() needs to be called explicitly or the user
can call evhttp_send_reply_end() which just frees the request, too.