syncbase: add hook to get service and DB stores (needed by sync)

Also moves internal interfaces to a separate package, hopefully
making the code structure easier to grok.

Change-Id: I449bfa9ed184334118dd28b2695d9792c7f43f56
diff --git a/services/syncbase/server/dispatcher.go b/services/syncbase/server/dispatcher.go
index 4a4c9e2..7f2fd1f 100644
--- a/services/syncbase/server/dispatcher.go
+++ b/services/syncbase/server/dispatcher.go
@@ -47,8 +47,9 @@
 	}
 
 	aExists := false
-	a, err := disp.s.app(nil, nil, appName)
-	if err == nil {
+	var a *app
+	if aint, err := disp.s.App(nil, nil, appName); err == nil {
+		a = aint.(*app) // panics on failure, as desired
 		aExists = true
 	} else {
 		if verror.ErrorID(err) != verror.ErrNoExistOrNoAccess.ID {