ε-ORC: enable multiple instances per function

[ccicconetti]

Currently, the APIs implicitly support a single instance per function, since the output_mapping field of PatchRequest maps each channel to a single InstanceId.

This must be changed to enable horizontal scalability within an orchestration domain.

Required changes include:

1. Updating Orchestrator so that it supports > 1 function instance.
2. Change the PatchRequest message so that output_mapping is a map from the channel name to a vector of InstanceId.
3. Update the callback table in the function instances so that they support multiple outputs, with load balancing based on a weight (to be added to the PatchRequest, too).