switch stateless actors with small overheads to tasks

[Joshuaalbert]

Some actors claim large resources for large swathes of time, but sit idle for lots of time as they wait for the deps to finish. This means we lose resource utilisation. Since the application of tasks is largely linear, we could refactor into a single actor that calls tasks. See visual below:

Currently if we have actors A (resources=X), B (resources=Y), C (resources=Z), linearly composed, A o B o C, then we claim X+Y+Z resources, even though only one is ever using compute at a given moment.

Instead we could reformulate as stitching together ephemeral tasks, freeing up resources in between.

Or, instead we could