Add unbuffered/unthreaded worker option

[kzscisoft]

For low metric send rate provide the user with the option to remove the buffer creation and multi-threaded dispatch of metrics. This will ensure metrics are sent directly where a queue is superfluous.

[wk9874]

When reviewing: Run all examples with direct dispatcher repeatedly, check for index error

Set run._dispatch_mode = 'prompt' (may change to direct)