euphoria-core: earlyTriggering of merging windows

[xitep]

Early triggering of merging windows is unsound! A merging window, e.g. session window, may change its identity over time. If such a window is fired multiple times while its identity changed, receiving downstream operators have no chance associating such multiple distinct windows as originating from one.

This typically causes problems for downstream operators which do access the window for business logic reasons. We may want to perform a validation as part of translating the flow DAG and either emit a warning or have the validation fail, if the user can access a merging window in a downstream operator.