Clarify interaction of UPC++ Serialization with Boost

[Dan Bonachea]

This task was deferred from pull request #20 for a future release that includes Boost serialization support:

More needs to be said about the UPC++ interaction with Boost. Specifically, I suspect that activating the Boost adapter for T means that when serializing an object of type T, the object and all sub-objects will use Boost serialization. Stated another way, our serialization calls into Boost, but Boost serialization does not call back to UPC++ (right?).

This matters if, for example, a given sub-object type implements both Boost-native serialization and UPCXX_SERIALIZED_FIELDS or some other UPC++ class serialization mechanism. The mechanism used at runtime will depend dynamically on whether serialization for that type is invoked directly by UPC++ or indirectly through the Boost adapter on a containing type.

This is especially relevant for std containers that may have both Boost-provided Boost-native serialization and runtime-provided UPC++ class serialization.

[Dan Bonachea]

This has been resolved in 2020.10.0 by removing the Boost serialization adapter from the normative library.

See Extras PR 25 for an experimental but functional Boost serialization adapter