Design & implement a "hello shards" program using sharded HoneyBadgerMPC

[sbellem]

This "hello shards" program should demonstrate the basics of using HoneyBadgerMPC "in sharded mode". It can be put under the apps/tutorial directory.

Initial thoughts on how to go about:

• Modify the current config mechanism to support/specify shards.
• Design and implement the intra-shard communication functionalities required.
• Design and implement the cross-shard communication functionalities required.

Configuration mechanism

A node should be configured with a shard id to identify which shard it belongs to. Nodes with the same id will belong to the same shard, and a deployment with a single shard should be equivalent more-or-less to a non-sharded deployment as it is right now. From a user perspective this means, that a user could omit specifying any shard id when deploying a non-sharded application. Internally this can perhaps be handled with a default shard id.

Although the key idea is perhaps simple, it may be worthwhile to briefly look into existing sharded systems to see how they handle the configuration.