store: Call connection.close in PerAccountStore.dispose

This seems hygienic. It makes sense to explicitly clean up HTTP
client resources when we know we'll be setting up a new client
instance to replace the old one. (The dead-queue reload is currently
the only path where PerAccountStore.dispose is called.)

It will also make sense to clean up these resources when an account
is logged out. That's a feature we'll be implementing soon, with
PerAccountStore.dispose in that new path as well.

The "TODO abort long-poll and close ApiConnection" was on
UpdateMachine's dispose method, not PerAccountStore's. But since
PerAccountStore is what owns the connection, its dispose method
seemed like the more appropriate place to close the connection.

lib/model/store.dart

@@ -464,6 +464,7 @@ class PerAccountStore extends ChangeNotifier with EmojiStore, ChannelStore, Mess
     _messages.dispose();
     typingStatus.dispose();
     updateMachine?.dispose();
+    connection.close();
     _disposed = true;
     super.dispose();
   }
@@ -980,7 +981,11 @@ class UpdateMachine {
   ///
   /// After this is called, the instance is not in a usable state
   /// and should be abandoned.
-  void dispose() { // TODO abort long-poll and close ApiConnection
+  ///
+  /// To abort polling mid-request, [store]'s [PerAccountStore.connection]
+  /// needs to be closed using [ApiConnection.close], which causes in-progress
+  /// requests to error. [PerAccountStore.dispose] does that.
+  void dispose() {
     assert(!_disposed);
     NotificationService.instance.token.removeListener(_registerNotificationToken);
     _disposed = true;