[ANN] moonpool 0.4


Moonpool 0.4 was released on opam a few days ago.

There are not a lot of new features, but Moonpool is now a better citizen wrt resource usage: if no pool is active, background domains will shut down (to be spun back again later if they’re needed). This was trickier to get right than I expected and a last bug was found during the release process and fixed with the help of @avsm, @dra27 and Mark Telvers).