refactor: Improve clearing

This makes sure that the database is null after clearing so it will
be built again using the databaseBuilder.
Also this makes sure that the sync has
aborted BEFORE the clearing starts to
get rid of some warnings in the logs.

lib/src/client.dart

@@ -912,6 +912,7 @@ class Client extends MatrixApi {
           }
           await legacyDatabase.close();
           if (migrateClient != null) {
+            _initLock = false;
             return init();
           }
         }
@@ -992,10 +993,15 @@ class Client extends MatrixApi {
   Future<void> clear() async {
     Logs().outputEvents.clear();
     try {
+      await abortSync();
       await database?.clear(id);
+      _backgroundSync = true;
     } catch (e, s) {
       Logs().e('Unable to clear database', e, s);
+    } finally {
+      _database = null;
     }
+
     _id = accessToken = syncFilterId =
         homeserver = _userID = _deviceID = _deviceName = prevBatch = null;
     _rooms = [];