added simple push device token validation

Ok. I guess if Apple or Google will decide (as an example) to generate 5k long device-tokens, we need to change the database constraint anyways....  But currently we don't test that it does not go beyond 4k, right ? (not for iOS nor for Android, which was the original reason we added the 4096 size DB constraint). See:
https://issues.jboss.org/browse/AGPUSH-541 (last comment)