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/sync/userstream.go

@@ -34,7 +34,7 @@ type UserStream struct {
 	// Closed when there is an update.
 	signalChannel chan struct{}
 	// The last sync position that there may have been an update for the user
-	pos types.PaginationToken
+	pos types.StreamingToken
 	// The last time when we had some listeners waiting
 	timeOfLastChannel time.Time
 	// The number of listeners waiting
@@ -50,7 +50,7 @@ type UserStreamListener struct {
 }
 
 // NewUserStream creates a new user stream
-func NewUserStream(userID string, currPos types.PaginationToken) *UserStream {
+func NewUserStream(userID string, currPos types.StreamingToken) *UserStream {
 	return &UserStream{
 		UserID:            userID,
 		timeOfLastChannel: time.Now(),
@@ -83,7 +83,7 @@ func (s *UserStream) GetListener(ctx context.Context) UserStreamListener {
 }
 
 // Broadcast a new sync position for this user.
-func (s *UserStream) Broadcast(pos types.PaginationToken) {
+func (s *UserStream) Broadcast(pos types.StreamingToken) {
 	s.lock.Lock()
 	defer s.lock.Unlock()
 
@@ -116,9 +116,9 @@ func (s *UserStream) TimeOfLastNonEmpty() time.Time {
 	return s.timeOfLastChannel
 }
 
-// GetStreamPosition returns last sync position which the UserStream was
+// GetSyncPosition returns last sync position which the UserStream was
 // notified about
-func (s *UserStreamListener) GetSyncPosition() types.PaginationToken {
+func (s *UserStreamListener) GetSyncPosition() types.StreamingToken {
 	s.userStream.lock.Lock()
 	defer s.userStream.lock.Unlock()
 
@@ -130,7 +130,7 @@ func (s *UserStreamListener) GetSyncPosition() types.PaginationToken {
 // sincePos specifies from which point we want to be notified about. If there
 // has already been an update after sincePos we'll return a closed channel
 // immediately.
-func (s *UserStreamListener) GetNotifyChannel(sincePos types.PaginationToken) <-chan struct{} {
+func (s *UserStreamListener) GetNotifyChannel(sincePos types.StreamingToken) <-chan struct{} {
 	s.userStream.lock.Lock()
 	defer s.userStream.lock.Unlock()