Atomic schedule

Here is a possible fix for #437.  It atomizes the `schedule=` method so the clear_schedules and set_schedule are all in one pipeline.
I think you'll want `Redis#multi` here, not `Redis#pipelined`; pipelining only ensures that your ruby process doesn't wait for the reply to each command before issuing the next one – which can speed things up but won't guarantee that the commands are atomic – while the `multi`...`exec` pattern ensures that all commands in between are executed on the redis server together.