replaced account data singleton with a session model

lib/api/request_cache.dart

@@ -1,7 +1,7 @@
 import 'dart:async';
 import 'dart:convert';
 
-import '../model/account_data.dart';
+import '../session/session_manager.dart';
 import 'api_response.dart';
 import 'cache_store.dart';
 import 'errors/parse_exception.dart';
@@ -47,7 +47,7 @@ abstract class RequestCache<T extends ApiResponse?> {
   static void ignore(Exception e) {}
 
   Future<void> start(String document) async {
-    final epoch = AccountData().sessionEpoch;
+    final epoch = SessionManager().sessionEpoch;
     try {
       final entry = await CacheStore.instance.read(document);
       var lastUpdate = entry?.lastUpdate ?? 0;
@@ -85,7 +85,7 @@ abstract class RequestCache<T extends ApiResponse?> {
       final newValue = await onLoad();
       // The cache is shared, so a late response of a signed-out
       // account would otherwise be cached for the next one.
-      if (!AccountData().isCurrentSession(epoch)) {
+      if (!SessionManager().isCurrentSession(epoch)) {
         onError(const StaleSessionException());
         return;
       }