Add version field serializer

[tesseract2048]

A version based serializer with backward compatibility (no forward compatibility).

This can be used for scenarios like:

• Already used FieldSerializer to serialize persisted object, but about to add field(s)
• Need light-weighted and simple backward compatible serializer with little overhead, since forward compatibility are not always necessary

[NathanSweet]

Could be int, since writeVarInt is used to write the version.

[NathanSweet]

I like the serializer idea! It has less overhead than TaggedFieldSerializer and the only downside versus TaggedFieldSerializer is that fields can't be renamed. Can you address the comments above and we'll merge?

[magro]

It would be great if the pull request would update the readme as well.

[tesseract2048]

Hi, thanks for attention.

Actually magic number worked in my scenario (because i somehow used FieldSerializer to store objects to HBase...), so i figured that this could work for someone else, but not necessarily (yes FieldSerializer could write the same number at the very beginning, with small possibility, though the lost could be endured).
So, in my case, using FieldSerializer for persisted objects are bad choice already, the best thing to do is try to fix with a simple approach (without refreshing the whole database, that could cost a lot.)

I will the clean up the code and update later.

[tesseract2048]

Additionally, i think forward compatibility could be achieved with appending version before every field, and use chunked i/o to support skipping.
But turns out that would introduce more overhead, and forward compatibility is rarely required in my view (they can just deploy all programs with latest version).

[tesseract2048]

Hi there, I believe the latest commit will do the work, since there is no need to introduce more overhead for forward compatibility.
And i've updated README.

[NathanSweet]

Thanks!