Testcase for filter with broadcast variable
Add test for StringSerializer for #589
Mixed key types for join
Minor error checks and message cleanups
Old Java Api now also uses the binary Union Operator and cleaned the common.api/optimizer level from input operator lists. Also added PropertyPropagationTest for new java api union
Comments and cleanup for (delta) iterations.
Clean up common api operators and marked var arg inputs as implicit unions as deprecated.
Record API builder pattern suppoerts the model still. The common api uses an explicit union model.
Post merge cleanup of Union operator.