Switch to using PUSHAUTO for multistream

[victorges]

As reported on #115, we sometimes get multistream crashing and not
reconnecting. This is by design, as Mist's PUSH abstraction is made to connect only once.

The fix is to switch to PUSHAUTO which handles the reconnect, and the only real complexity here (apart from
changing the object type) is making sure that we delete these pushes when the stream ends, otherwise they will leak
in mist (and if stream restarts even get duplicated and cause a mess).