Support multiple write destinations by logger.

[georgolden]

Problem

We need to write some logs to a db (postgres/redis, etc.) and be able to write logs by meaning. Write operation log to a specific table in a database, write statistic logs to a different table, etc.

Solution

Provide writable destinations by LoggerOptions from config file. Writable destination must have method write(buffer, fn) similar as NodeJS Writable Stream. Logger must support multiple write destinations like a set of writables.