Verify thread safety of JavaScript based editors

[kipz]

From @johnsonr on November 22, 2016 19:56

Need to check isolation between "edit" methods invocations, even if not state is shared.

[kipz]

With the move to property injection, we'll need to create a new instance of each rug each time we run an operation, as this definitely wouldn't be thread safe. In any case, this should be fast if we reuse the ScriptEngine instance.

#229

NOTE: Each thread must have its own bindings!