Better split between the cleanups from self and the ones from the kernel (ie on maest...

Better split between the cleanups from self and the ones from the kernel (ie on maestro context)

This shall remove the need for OS synchronization when registering the
actors that should die. So that's hopefully another fix for the problem solved
by be00fecb16c89d44253052889f4815b35ab9abbe

Also, it protects more modifications to global datastructures from
concurrent modifications by doing them in maestro context. The
previous solution was only protecting a very small amount of them.