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build reference documentation with asciidoc

 - "gradle asciidoctor" works
 - chapters are empty
Fix failing tests in DataflowStream.
It looks like it's a more general problem with switching to Java 8, because the same code works fine with previous versions of Java.
RemoteDataflowVariable extracted to separate package
RemoteDataflowVariable test
Introducing RemoteDataflows
DataflowVariable can be published
DataflowVariable can be retrieved (locally). Updated javadoc
DataflowVariable can be retrieved (remotely). Updated javadoc
RemoteDataflows is an util class.
Retrieving DataflowVariable from remote host returns future
Retrieving DataflowVariable from remote host
Retrieving DataflowVariable from remote host
Retrieving DataflowVariable from remote host
Retrieving DataflowVariable from remote host - quick workaround.
Retrieving DataflowVariable from remote host - test.
Retrieving DataflowVariable from remote host - test.
Retrieving DataflowVariable from remote host - demo.
Retrieving DataflowVariable from remote host - binding tests.
Example remote DataflowVariable - refactor