network: disable_recv before telling actor to close connection

As of d62ad96, when the connection can't receive more data from the
client, it tells the actor to stop the connection and calls
disable_recv(). The actor operates in it's own thread and when it stops
the connection, disable_recv is being called again from a different
thread. Since the actor is told to stop the connection before
disable_recv is called, the two calls to disable_recv may happen
simultaneously.

This causes a race condition issue where both threads can reach past the
check that recv_id is not None before either of them set it to None. If
one of them set it to None before the other one tries to use it, an
error is raised.

This commit calls disable_recv before telling the actor to stop the
connection. Since disable_recv is a blocking call, this ensures that
recv_id is being set to None before the actor thread begins to stop the
connection.

Fixes #781
I can't guarantee it either, but I think this fixes the issue caused by d62ad96 at least. After this change, the actor thread would stop the connection, and could get to disable_recv at the same time as the thread receiving the callback in Connection got there. With this change, disable_recv is done (blocking) first, so when the actor thread is told to stop the connection, recv_id is always None.

Though, if the connection can handle multiple callbacks at the same time, it may still be a problem where two callbacks try to stop the connection and get to disable_recv at the same time. But, as you say, this is (much) less likely than the above issue, and I don't think I have run into this myself.