Sync IPC interface

[NikVolf]

[x] sorting out with multiple handshake structures spawned for each service in codegen - #1586

[coveralls]

Changes Unknown when pulling 0555b9a on sync-ipc-rpc into * on master*.

[tomusdrw]

That's probably for a next PR but can we replace the build scripts with something like:

fn main() {
  let paths = HashMap::new();
  ethcore_ipc_codegen::build(paths);
}

This will allow us to have just codegen as build dependency, without any syntex stuff. Might be also much easier to keep it in sync in future.

[NikVolf]

yep i will do it when will be sorting things with compiler plugins

[gavofyork]

basic style and premise lgtm.refctoring of @arkpar 's code though...

[tomusdrw]

Seems ok for me too. I don't feel I have enough knowledge to decide, though.

[coveralls]

Changes Unknown when pulling 3ec78b5 on sync-ipc-rpc into * on master*.