add support to scala 2.12 and flink 1.8.x

[123avi]

It seems that there are significant changes in state handling and api of some of the methods in flink 1.8

[juhoautio]

It seems that Bravo is not actively developed anymore as the feature set has made it into flink with the State processing API. I will try to migrate to that instead, as that is the way forward.

https://ci.apache.org/projects/flink/flink-docs-stable/dev/libs/state_processor_api.html