Add explicit logging interface

Purpose
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Previous version of logger provided only global functions what is inconvenient in case if you need to operate an interface object.

Implementation
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This commit introduces explicit `Logger` interface, changes all the loggers to the `Logger`, introduces `GetLogger` - global logger while preserving the original global functions.

@r0mant @jeremyschlatter please review.
To save another allocation you could pass the Writer to makeMessage and call fmt.Fprintf in makeMessage.