Engine API: define versioning and set initial versions

[mkalinin]

Defines a versioning mechanism for methods and structures and sets initial versions.

[djrtwo]

looks reasonable. I'd like to get some input from @holiman or others that took part more in the convo at amphora

[mkalinin]

cc @lightclient, @protolambda

[ralexstokes]

there are a number of ways to handle this and they all have different trade-offs...

in the interest of not bike-shedding, i think versioning the JSON-RPC method name itself is fine.

unless someone else can chime in and express i strong preference otherwise, i say we merge this!

[holiman]

Yeah, I guess this is reasonable. It's not beautiful, but I guess it's what we agreed.

[protolambda]

I wish the JSON-RPC had better versioning options, but this works. Versioning an API from the start is the best kind of versioning, LGTM.