constants: add addressVersion. useful for connecting to testnet.

[chjj]

No description provided.

[indutny]

I don't think that there is a point of having this, unless we fully support testnet. This should require adding a lot of new constants, or even splitting the constants module into two.

[chjj]

I've implemented testnet in my own module. The only thing that's a pain to change from the "outside" is this value since it's hardcoded into a function and testnet uses 111 rather than 0. It's not as much work as you might think (different seeds, different checkpoint blocks, different port, different address version - sounds like a lot, but it's not that much) But I understand if you don't want to.

[indutny]

What about magic value used for framing?

[indutny]

How are you changing it in a module?

[chjj]

What about magic value used for framing?

I forgot about that one. Fair enough, but it's only a single line:

bcoin.protocol.constants.magic = 0x0709110b;

[chjj]

Nevermind, it's not a big deal. I have too many PRs open as it is.