This PR tries to solve the issues raised by redis#732 regarding
the danger of disconnect clients from the `ConnectionPool.disconnect`
method executed by a Thread different that those ones that are in charge
of the connections.

Instead of call the `Connection.disconnect` method it uses the syscall
`shutdown` to leave the socket unusable. Once the connection tries to use
the socket, even when it is already blocked such us the `PubSub` pattern, it
gets a `socket.error` exception that will be cactched by the
`Connection` class to then raise an `ConnectionError` and disconnect the
socket in a clean and safe way.

The `Client.execute_command` function catches the `ConnectionError` exception
and tries to connect again and run the command that raised the error.
Worth mentioning that in the case of the `Sentinel` environment, if some
changes regarding the Redis pool of servers - perhaps the mater went
down and a slave was promoted - the next command will be executed using
an other server.