MEDIUM: connection: don't stop receiving events in the FD handler

The remaining epoll_ctl() calls are exclusively caused by the disagreement
between conn_fd_handler() and the mux receiving the data: the fd handler
wants to stop after having woken up the tasklet, then the mux after
receiving data wants to receive again. Given that they don't happen in
the same poll loop when there are many FDs, this causes a lot of state
changes.

As suggested by Olivier, if the task is already scheduled for running,
we don't need to disable the event because it's in the run queue, poll()
cannot stop, and reporting it again will be harmless. What *might*
happen however is that a sampling-based poller like epoll() would report
many times the same event and has trouble getting others behind. But if
it would happen, it would still indicate the run queue has plenty of
pending operations, so it would in fact only displace the problem from
the poller to the run queue, which doesn't seem to be worse (and in
fact we do support priorities while the poller does not).

By doing this change, the keep-alive test with 1k conns and 100k reqs
completely gets rid of the per-request epoll_ctl changes, while still
not causing extra recvfrom() :

  $ ./h1load -n 100000 -t 4 -c 1000 -T 20 -F 127.0.0.1:8001/?s=1k/t=20

  200000 sendto 1
  200000 recvfrom 1
   10762 epoll_wait 1
    3664 epoll_ctl 1
    1999 recvfrom -1

In close mode, it didn't change anything, we're still in the optimal
case (2 epoll per connection) :

  $ ./h1load -n 100000 -r 1 -t 4 -c 1000 -T 20 -F 127.0.0.1:8001/?s=1k/t=20

  203764 epoll_ctl 1
  200000 sendto 1
  200000 recvfrom 1
    6091 epoll_wait 1
    2994 recvfrom -1

src/connection.c

@@ -106,7 +106,6 @@ void conn_fd_handler(int fd)
 				conn->subs = NULL;
 		} else
 			io_available = 1;
-		fd_stop_send(fd);
 	}
 
 	/* The data transfer starts here and stops on error and handshakes. Note
@@ -126,7 +125,6 @@ void conn_fd_handler(int fd)
 				conn->subs = NULL;
 		} else
 			io_available = 1;
-		fd_stop_recv(fd);
 	}
 
  leave: