This is an optimization to reduce the payload size and time spent by the debugger.

stackdriver_debugger 0.2.0 adds an id field for captured variables (arrays, strings, objects). Implemented here.

If provided, use that id to de-dup variables in the VariableTable (previously we used spl_object_hash which is only available for objects).

This is useful for large arrays (like $_SERVER) so that we don't need to have multiple copies of the same nested data structure when reporting variables to stackdriver.