Refactor MMS send/download to be synchronous.

1) Make the radio change a synchronous action with a timeout.

2) Move the send logic into an MmsTransport, in preparation for
   UniversalTransport composition.

3) Move the download logic into a synchronous receiver.

src/org/thoughtcrime/securesms/database/ThreadDatabase.java

@@ -349,7 +349,7 @@ public class ThreadDatabase extends Database {
     }
   }
 
-  public Recipients getRecipientsForThreadId(Context context, long threadId) {
+  public Recipients getRecipientsForThreadId(long threadId) {
     SQLiteDatabase db = databaseHelper.getReadableDatabase();
     Cursor cursor     = null;