ReleaseFuncs should be non-blocking

[zenhack]

I discussed this with @lthibault on matrix: It is unintuitive, awkward, and I think even undocumented that the ReleaseFuncs returned by method calls will actually block until the method call returns. Instead, I think they should never block and if a method has not already returned, arrange for the promise to be released when it does return, but don't wait for it.

The trivial implementation of this would be to just fork a goroutine in the body.