Core: Revise the async mechanisms

[cristi8]

We have a number of parallel mechanisms in place for faster serialization and server communication.

We should make sure the appropriate async mechanisms is used in each case.

In particular:

• calling .Result on a task will block the calling thread (used in a couple of places, make sure it's intended)
• ensure (de)serialization context switching and waiting is done properly
• never block the calling thread of operations.receive / etc (might be UI thread) (issue described here: https://speckle.community/t/speckle-core-cant-receive-stream-data-on-specific-machine/2409/7 )

This ticket is a result of multiple related issues / questions.

We should also briefly document somewhere the parallelism involved and how it's done.

[cristi8]

Modified a few .Result / .Wait with await, but there are still a few places, like the Transport sync method:

public string GetObject(string id)

Still has to block the calling thread and can deadlock UI calls

So for now, callers of the SDK, especially when calling methods from the UI thread, must call with Task.Run to switch to a background thread for the actual work