Create shard-side handlers for basic messages
Implement the handlers for CreateVertices, CreateEdges and ScanAll. Use
or modify the defined messages to interact with individual Shards and
test their behavior. Shard is currently being owned by ShardRsm
instances. The two top level dispatching functions Read() and Apply()
are responsible for read- and write operations respectively. Currently
there are a handful of messages that are defined but not utilized, these
will be used in the near future, as well as a couple of handler
functions with empty implementations.
