[RPC] Introduced primitive<T> protocol template.

In order to simplify the RPC subsystem for service methods which only return a value and close the
stream, a new protocol is introduced - primitive<T> based on two mutually exclusive final messages
value<T> and error.
[RPC] Locator snapshot generation IDs.
Merge pull request #137 from cocaine/primitive-protocols

[RPC] Introduced primitive<T> protocol template.