setGlobalPropagator/getGlobalPropagator for each propagator type

[srikanthccv]

This is probably not a big issue. The current setGlobalPropagator and getGlobalPropagator only accept the TextMapPropagator type but it is possible that there can be other propagator types such as binary propagator. We can introduce the new global set/get methods for new propagator types to solve that problem. It would be nice if the names are more explicit to indicate what they do setGlobalPropagator -> setGlobalTextMapPropagator and getGlobalPropagator -> getGlobalTextMapPropagator.