Change CamusAvroWrapper's signature back to public static

@HenryCaiHaiying Sorry for the careless, I've searched the usage of CamusAvroWrapper and just found one in KafkaAvroMessageDecoder, so I just changed the signature of CamusAvroWrapper recklessly.

PS. Will you guys provide code formating configurations(for intellij or eclipse)? Just found camus match none of my existed ones :)
Thanks for the work.
Can you generate the against the latest code (where I already reverted you previous change).
And we probably don't really need a separate 'getCustomTimestamp' method, the logic probably can just embed inside 'getTimestamp'.