event callbacks that make gRPC calls can cause race conditions

[randallfrank]

There are two potential issues here:

1. if a callback makes another implicit gRPC call, it can cause re-entrancy problems (e.g. the gRPC connection can be shut down)
2. repeated callbacks can queue client side and cause gil contention in the client Python

In any case, the current implementation can cause problems with calls like this:

session.ensight.objs.core.parts[("hood", "engine")].set_attr(session.ensight.enums.VISIBLE, False)

for a class-based callback associated with the "VISIBLE" attribute. The current work-around is to use macro expansion in the callback string to avoid the conflict in most cases.