syncapi: Rename and split out tokens (#1025)

* syncapi: Rename and split out tokens

Previously we used the badly named `PaginationToken` which was
used for both `/sync` and `/messages` requests. This quickly
became confusing because named fields like `PDUPosition` meant
different things depending on the token type. Instead, we now have
two token types: `TopologyToken` and `StreamingToken`, both of
which have fields which make more sense for their specific situations.

Updated the codebase to use one or the other. `PaginationToken` still
lives on as `syncToken`, an unexported type which both tokens rely on.
This allows us to guarantee that the specific mappings of positions
to a string remain solely under the control of the `types` package.
This enables us to move high-level conceptual things like
"decrement this topological token" to function calls e.g
`TopologicalToken.Decrement()`.

Currently broken because `/messages` seemingly used both stream and
topological tokens, though I need to confirm this.

* final tweaks/hacks

* spurious logging

* Review comments and linting

syncapi/consumers/clientapi.go

@@ -90,7 +90,7 @@ func (s *OutputClientDataConsumer) onMessage(msg *sarama.ConsumerMessage) error
 		}).Panicf("could not save account data")
 	}
 
-	s.notifier.OnNewEvent(nil, "", []string{string(msg.Key)}, types.PaginationToken{PDUPosition: pduPos})
+	s.notifier.OnNewEvent(nil, "", []string{string(msg.Key)}, types.NewStreamToken(pduPos, 0))
 
 	return nil
 }