api: use id strings instead of Info refs in signals

If signals are to be consumed from a queue, then at least one copy
is required, and potentially beefy custom types shouldn't be
marshalled between threads without good reason. It is more sensible
to use conversation Id parameters, as consumers can likely make
good use only the Id, and may query for an Info structure at any
time if needed.

Change-Id: I5151c8ac006857c6510258332647e52ce16821af