Optimize py:set by batching

[LaCuneta]

Currently, every py:set sends a message to the python process and waits for a result. However, we could instead batch py:sets with py:runs and py:runresults to reduce the amount of communication. This would likely be a pretty good performance boost, as communication time is definitely a big bottleneck.

Copied from: https://github.com/qiemem/PythonExtension/issues/10