Fix broken assignment operator.
Merge pull request #120 from 3Hren/patch-2

Fix broken assignment operator.
Synchronized with Blackhole v0.2.

 - There is no longer need for explicit logger synchronization. Logger is thread-safe by default.
 - Cocaine should no longer format debug messages with `0` verbosity.
 - Less code needed for conversion between `dynamic_t` and Blackhole's internal objects for external configuration.