Add version and active flags to series

Once this has been merged I'll rewrite the serialization tests in a way similar to https://github.com/monetas/btcwallet/pull/50
I don't think anyone interested in the RPC API would look there anyway. It's purely to make it clear to anyone who looks at the comment and sees the active field where it's currenly in use or not.

From that perspective your suggestion makes sense as well, so I'd be fine with that.