Signal-with-start support

[iand675]

We had most of the support for this in place already, but the tricky bit was that we had to move Workflow execution away from a 1:1 receive commands from Temporal server -> flush commands to Temporal server model. We actually need to exhaust the activation queue in its entirety before flushing results up to the server. Switching to away from Control.Concurrent.Chan to Conctrol.Concurrent.STM.TQueue gives us the necessary primitives to do this properly.