Extend example/tests/async.rs to offload message creation/processing to separate tasks

[fjarri]

This will demonstrate the target usage pattern for a heavy protocol.

Also I have a suspicion that we won't actually be able to finalize() because it consumes the Session object, and references to it are being sent to tasks, so we will have to wait for all of them to finish, which we won't want to do if there's a threshold needed to finalize (see also #11). So we will need to think of some different API to support this case.

[fjarri]

As evidenced by entropyxyz/entropy-core#1136, there don't seem to be any problems with parallel processing. But it raises another question: how do we test the performance gain from it in a real protocol? Should run_sync() optionally offload message processing to threads?