BUG/MEDIUM: connection: stop polling for sending when the event is ready

BUG/MEDIUM: connection: stop polling for sending when the event is ready

With commit 065a025610 ("MEDIUM: connection: don't stop receiving events
in the FD handler") we disabled a number of fd_stop_* in conn_fd_handler(),
in order to wait for their respective handlers to deal with them. But it
is not correct to do that for the send direction, as we may very well
have nothing to send. This is visible when connecting in TCP mode to
a server with no data to send, there's nobody anymore to disable the
polling for the send direction.

And it is logical, on the recv() path we know the system has data to
deliver and that some code will be in charge of it. On the send
direction we simply don't know if it was the result of a successful
connect() or if there is still something to send. In any case we
almost never fill the network buffer on a single send() after being
woken up by the system, so disabling the FD immediately or much later
will not change the number of operations.

No backport is needed, this is 2.2-dev.