Fixes problem Adam reported re: flag watcher failing

Problem was due to accumulation of messages marked for removal on
local side not being (eventually) removed from MessageLocationTable.
Can happen if folder is closed before EXPUNGE server data arrives,
or if Geary is shut down or crashes before same.  Folder
normalization will deal with situation where locally removed
message was not actually removed on server.

src/engine/sqlite/api/sqlite-folder.vala

@@ -73,6 +73,13 @@ private class Geary.Sqlite.Folder : Object, Geary.ReferenceSemantics {
             throw new EngineError.ALREADY_OPEN("%s already open", to_string());
         
         opened = true;
+        
+        // Possible some messages were marked for removal in prior session and not properly deleted
+        // (for example, if the EXPUNGE message didn't come back before closing the folder, or
+        // the app is shut down or crashes) ... folder normalization will take care of messages that
+        // are still in folder even if they're marked here, so go ahead and remove them from the
+        // location table
+        yield location_table.remove_all_marked_for_remove(null, folder_row.id, cancellable);
     }
     
     public async void close_async(Cancellable? cancellable = null) throws Error {