Switch to using PUSHAUTO for multistream

As reported on https://github.com/livepeer/stream-tester/issues/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).

Start by finding the multistream PUSH creation and stream-end cleanup paths, then inspect how the stream tester represents and deletes Mist pushes. Confirm the PUSHAUTO behavior for reconnects and verify that ending and restarting a stream does not leave duplicated pushes.

Written by the indexing model from the issue text.