Support for kafka and multiple sinks

What's Changed

• Refactor encode2 function by @dmussaku in #1
• Improve tests avro encode tests by @makarski in #3
• Avro encode: extend type coverage by @makarski in #4
• Add bson to avro support for decimal data type by @makarski in #5
• Improve logging and error handling by @makarski in #6
• DB listener: add concurrency support by @makarski in #7
• Changed bson to avro conversion to support schema context by @makarski in #8
• Add avro union type support by @makarski in #9
• Fixed avro encoding to binary by @makarski in #10
• Remove test dev debug features: subscribe, persist, etc by @makarski in #11
• Add support for kafka sink, multiple sinks for source by @makarski in #14
• Clean up sample config by @makarski in #15

Full Changelog: https://github.com/makarski/mstream/commits/0.6.0