Dynamically update workflow

[TaoXieSZ]

I am implementing this change to help goflow dynamically update the DAG.

First, the flowRuntime uses requestID as key to store a "snapshot" of flow handler in fRuntime.ExecFlows
Then, the gcWorker removed finished flow in ExecFlows periodcally.

[s8sg]

@TaoXieSZ Thank you for the PR.

Dynamic Update

You don't need a separate functionality to update flow. Your flow definition function gets called every time a node executes.

Can you provide me an use-case that you are trying to achieve - I might be able to share an alternative approach to achieve the same

GC

I like the GC approach. Currently whenever a request is finished, flow - we are cleaning up the states. Current implementation will fail given the request would itself cleanup the states and then the GC would try to clean it again
Same can happen due to a race condition is when the GC has cleaned the state, then request execution try to clean it

Also keeping it in memory is not an option - this should be read from the redis.

ExecFlows               map[string]FlowSnapshot

I'll suggest to create issues first with the problem you are trying to solve and we can discuss accordingly
I'll close this PR for now and would reopen if needed.