plugins/driver: update driver interface to support streaming stats

client/tr: use streaming stats api

TODO:
 * how to handle errors and closed channel during stats streaming
 * prevent tight loop if Stats(ctx) returns an error

drivers: update drivers TaskStats RPC to handle streaming results

executor: better error handling in stats rpc

docker: better control and error handling of stats rpc

driver: allow stats to return a recoverable error

client: fingerprint driver plugins