Message with verbosity 0 leads cocaine-runtime

to segfault as nullptr is sent to a stream.
Merge pull request #110 from noxiouz/fix_convert_verbosity_more_gently

Message with verbosity 0
Merge pull request #112 from 3Hren/master

Logging Improvements
Log message wording.
Beautifulization.
Factor out the defaults.
Create logs using the Context functions.
Fixed a couple of log messages.
Refined log messages in the Repository.
Do not try to move const references.
Amagad.
Report a service name in case of unsuccessful resolving.
Merge pull request #113 from noxiouz/make_locator_more_talkative

Report a service name in case of an unsuccessful resolving
Update locator.cpp
Persistent Raft state.

Now Raft stores own state in a persistent storage. This commit should fix the following bug: a node may vote twice in the same term, when it restarts and forgets about the previous vote.
Fix a data race and segfault.

* Configuration machine uses proper reactor now. Before I accidentally passed wrong reactor to it, so my single-threaded code runned in two threads actually.
* Don't remove service client from the callback. As I know, most Cocaine IO objects shouldn't be deleted from the callbacks called by them. The service client is not an exeption from this rule.
* Unbind a readable_stream when an socket error occurres. It's because we don't want receive any events and anything with an invalid stream.
Don't send heartbeat if an append request is already scheduled to be sent.
Fixed serialization of raft::command_result.

Command_result should provide address of the actual leader, but I forgot to serialize it.
Some comments and log messages.
Don't cancel just posted error.

It was a good idea to stop watching a socket when a socket error occurs.
But it's a bad idea to cancel the callback which should notify about the error.