Contract migrate \ update naming

[alexvanin]

All contracts besides NNS have Migrate method that call Update method of native management contract. NNS contract use Update method. Migrate name origins from timer.go example.

It will be a bit more convenient to have the same signature for contract update methods, I guess. Maybe it will be better to use Update in all contracts because it corresponds with management contract call. Thoughts?

/cc @fyrchik @roman-khimov @AnnaShaleva

[AnnaShaleva]

I don't see something that prevents us from renaming, and I also consider it's good to have unified methods naming that follows the Management's one. So I vote for Update.

[roman-khimov]

I think Migrate comes from Neo Legacy where it did have some meaning, data needed to be moved from one contract (with old hash) to another (with new hash). Now Update is more correct for N3 because contract's storage or hash doesn't change, it's just an update of code and/or manifest. @AnnaShaleva, we can do the same renaming in neo-go for consistency.