[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.
[RPC] Make protocol tags ephemeral again.
[Source] Dockerized.
[Source] Some dockerization improvements.
[Source] Bump README.
[ASIO] Refactored some code for Actors.
[General] Prettifying.
[Locator] Do not send empty snapshots.
[Cluster] Static cluster configuration.
[Source] Comments!
[Source] Codestyle.
[Source] More codestyle.
[Cluster] Added more diagnositics to Predefined.
[Cluster] Use `interval` argument for real.
[Source] Changelog.