Better parallelism with session-async.el

[m-fleury]

Use https://codeberg.org/FelipeLema/session-async.el instead of async. This has several advantages:

• we can reuse sessions instead of using a single one (should be faster because it reduces the start-up cost)
• we can interrupt sessions instead of letting them finish (useful for goal printing)
• solve the problem of leftover processes

If I see it right, fixing it consists in adding a new variable to hold the session name, start the session at the beginning (loading the required functions instead of reloading them every time), and replace the async by their counterpart (with the backquotes).

[m-fleury]

The file is already too complicated, so

• let this just be a drop-in of the async.
• simplify the code by removing the duplicated code for printing and use a 'require' instead