4.x: Add ExceptionHelper & HalfSerializer lock-free tools

[akarnokd]

This PR adds two lock-free helper class to work with partial signal serialization and atomic exception management.

HalfSerializer

This is an efficient, lock-free serialization when the OnNext are still coming in sequentially but there could be a concurrent OnError or OnCompleted happening, for example, with TakeUntil or SkipUntil. These methods establish the correct sequential behavior.

ExceptionHelper

Hosts a shared termination indication exception (similar to how BooleanDisposable.True is used for indicating a disposed IDisposable field).

In addition, it offers methods for atomically setting one Exception, swapping in a the terminal exception and getting back the last exception and the ability to aggregate exceptions until a terminal exception is swapped in. This latter feature can come in handy with future SelectMany and Concat operators that want to delay errors from the inner sequences and keep processing them, then finally emitting all the exceptions gathered as an AggregateException.

[danielcweber]

Pretty awesome concept.