Buffer payloads asynchronously when appropriate

[mattjohnsonpint]

During serialization, we buffer payloads of EnvelopeItems into memory so we can calculate lengths to include in headers.

As part of #1965 (released in 3.22.0), I switched to synchronous buffering of these payloads - because serialization of a fully materialized object to a MemoryStream gains no advantage by being async, and doing so adds some minor overhead.

That is still true, however I didn't consider that not all payloads are fully materialized. In particular, a StreamSerializable could be representing an attachment, in which case it would be reading from a FileStream or any other type of Stream (depending on how it was attached). Also, we may need other types of async materialization in the future.

Thus, we should defer to synchronous buffering only when we know the object is fully materialized - which is what our JsonSerializable class represents.

[SeanFeldman]

@mattjohnsonpint, it looks like there's a lot of work taking place with MemoryStream, and this is a hot path. Would there be any benefit in taking advantage of RecyclableMemoryStream?

[mattjohnsonpint]

@SeanFeldman - RecyclableMemoryStream looks useful indeed. Created #2299 to track. Thanks!