Add async_trait to SourceClient and TargetClient (#123)

* Use async_trait for SourceClient

* Use aync_trait for TargetClient

* Revert async_trait usage for Source/Target client

This reverts commit f636ffaffd60197e90e887362b4a0c35a0dc5a6c.
This reverts commit 2c15755e8c93318f8e0a605852efe87d72edb769.

I'm having a very hard time finding out what is causing compilation
issues, and I think it's best to start over again.

* Use async_trait for TargetClient

* Use async_trait for SourceClient

* Move where non-async methods are

* RustFmt

* QueuedHeader holds Arc to actual data

* Clean up async return type

Co-authored-by: Svyatoslav Nikolsky <[email protected]>

* Clean up async return type

Co-authored-by: Svyatoslav Nikolsky <[email protected]>

* Clean up async return type

Co-authored-by: Svyatoslav Nikolsky <[email protected]>

* Remove unused import

Co-authored-by: Svyatoslav Nikolsky <[email protected]>