euphoria-flink: support object re-use enabled

[xitep]

As part of #98 we disabled the experimental support for object re-use on the flink executor. We would like to reintroduce it again with the corresponding problems resolved. Part of the problems are states which are kept in memory and should be replaced to an external storage. Another problem is to ensure that the code base follows the rules stated by the flink documentation. (It should be noted local execution doesn't always reveal potential problems.)