feat: update dummy plugin to handle concurrent queries

[j-lanson]

This PR updates the dummy_rand_data plugin binary to support multiple active queries (mostly mirroring what was done for hipcheck/plugin in #297 ) .

Key changes:
engine.rs - added async_query, an async version of the salsa memoized query() function. This does not memoize, its just a proof of concept that we would be able to perform recursive calls to an async function.
main.rs - updates cmd_plugin() to actually spawn multiple simultaneously queries (using async_query, with a commented-out version demonstrating how to do so with sync query())
hipcheck_transport.rs - does receiver multiplexing
main.rs - RandDataRunner now spawns off a task that handles the individual query, giving it a cloned version of the HcTransport object for communicating over the gRPC channel.