Reduce memory leak by sharing shadowed globals by all script functions

[#60338690]

E.g. in:
http://lab.dev.concord.org/interactives.html#interactives/basic-examples/atom-data-table.json
memory consumption is redruced from 100MB to 30MB.

Now each interactive update (using Interactive Editor) increases
memory consumption by 1.5MB instead of 80MB.

It's nice, but I'm afraid the real bug is still there, it just makes
symptoms less serious.