Create single-shot Prefect Agent

[jacksund]

Similar to how a Fireworks worker will grab a single workflow, submit it, and then shutdown once the workflow completes, we'd also like to replicate this with a LocalAgent in prefect.

Currently, I think the best way to do this is by overwriting LocalAgent's _get_ready_flow_runs method and changing the following line:

# original line
flow_run_ids = set(result.data.get_runs_in_queue.flow_run_ids) 

# new code that reduces this query to a single flow run
flow_run_ids = set(result.data.get_runs_in_queue.flow_run_ids)
flow_run_ids = set([flow_run_ids.pop()]) if flow_run_ids else flow_run_ids  # new line

We'd then run LocalAgent with max_polls=1. This Agent will also automatically shut down without running anything if no flow runs are available.

[jacksund]

Rather than hacking this method, I'm also proposing more useful changes to Prefect's api here

Ideally, I'd be able to use an agent like so:

from prefect.agent.local import LocalAgent

LocalAgent(max_polls=1, max_runs_per_poll=1) 

When starting this agent, it would submit 1 flow run and then shutdown upon that flow's completion.

[jacksund]

added in #190