Merge pull request #136 from 3Hren/master

[Bug Fix] Fixed race condition in slave session.
[General] Tiny tweaks.
[RPC] More support for extension protocols.

Added extension protocols support to dispatches, message queues and traversal.
[RPC] Prettifying.
[ASIO] Do not die on fd exhaustion.

Attaching a new client to an engine involves dup(2) call, which might end up returning an invalid
file descriptor if maximum open files limit is reached. Handle that situation gracefully.
[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.