Support versioning of endpoints

[rnett]

We should somehow support explicit versions for endpoints, i.e. getItem/v2/, and perhaps for servers as well. Overloading handles this implicitly when signatures change, but when only the behavior changes, problems start. The actual method will have to change anyways for Kotlin's sake, so perhaps the best solution here is adding a name field to KrosstalkMethod.

[rnett]

There's another wrinkle here: old versions shouldn't be exposed on the client. Perhaps the best solution is to use #32, have a krosstalk for each version, and include the old ones into the current one. This works well for server version, but not so much for endpoint versioning. Could old versions just have their client methods deleted, or deprecated, and I don't have to deal with it? Requires having server-only methods.