Add LaunchMultiTask rpc interface for executor

[yahoNanJing]

Which issue does this PR close?

Closes #209.

Rationale for this change

What changes are included in this PR?

Are there any user-facing changes?

[yahoNanJing]

Hi @askoa, it's a draft PR for issue #209. To make it work, we need to refine something of the ExecutionPlan to split out mutable things, like task level metrics and statistics.

[askoa]

hey @yahoNanJing. Not sure why you have tagged me in this!

Edit: Thanks for the info btw.

[yahoNanJing]

Hi @askoa, I think you may be interested in this feature and can work on this PR together.

[yahoNanJing]

Hi @thinkharderdev, @avantgardnerio and @andygrove, could you help review this PR?

Since the scheduler part is under big change in another PR for task error recovering, I will propose another PR for the scheduler to invoke this new added grpc interface for multiple tasks launching after that PR finishes.

[askoa]

Hi @askoa, I think you may be interested in this feature and can work on this PR together.

@yahoNanJing Thanks for your response. I'll pick up issues from the github issues tab as time permits. I am working on something else now and does not have time to work on the PR at the moment.

[thinkharderdev]

LGTM