This change removes the opendc-core module. This module was an artifact
of the old codebase and remained mostly unused. This change removes all
usages of the module and if necessary introduces replacement classes.

This change adds more methods for controlling the lifecycle of Server instances.

This change extracts the API for the OpenDC Compute service into a separate
module to establish a clearer boundary between the interface meant for
consumers and interfaces meant for the the serve implementation.

This change converts the Server data class which can be used as a
stateful object to control an instance running in the cloud.

This change converts the low-level workload model to be pull-based. This
reduces the overhead that we experienced with our previous co-routine
based approach.

This change updates the workflow service to delegate the resource
scheduling logic to the virtualized resource provisioner.