How timely - I, too, got frustrated by the fact that org-babel src block evaluation is synchronous, so I built ob-async [1]. Rather than target a specific babel language, it aims to support any flavor of src block. It uses jwiegley's async [2] to execute the block in a second Emacs process and then inserts the results in the buffer when complete. There are still some rough edges (no :session support, for example), but it's worked well for my somewhat limited use cases so far.

[1] https://github.com/astahlman/ob-async [2] https://github.com/jwiegley/emacs-async