Improvements to workflows v2

[rafael]

Desc

The following PR handles all the issues described in: #4117

I think I still need to update some tests, but wanted to open it so we can start taking a look at the overall changes.

It looks big but it's mostly cosmetic changes. Notable changes are:

• Updates existent HorizontalResharding workflow to be explicit about the source and destination shards to be split/merged.
• Extends enable approvals in HorizontalResharding to have more granularity. After this change, users can decide explicitly which phases they want to require enable approvals.
• Adds a meta workflow that create an horizontal resharding workflow for all the available overlapping shards.
• Adds create_time to workflow protobuf so we can sort deterministically.

Example

The following is an excerpt of an horizontal resharding from 2 to 4 shards. Only the migrate serving types require approval and the generated workflows are started automatically.

It's a bit hard to see the details, but you can see that now it creates two workflows, one for each shard that is being split spilt.

[jvaidya]

This looks good to me on the whole and I think this will be a big improvement for resharding workflow when you are resharding a large number of shards. One question: I do not see you validating the sourceShards and targetShards input parameters. In the case where a human is putting together a SplitDiff or a SplitClone commandline, it would be good to validate these.

[rafael]

@jvaidya oh good catch. Will add that validation.

[jvaidya]

LGTM. Thanks for tackling this.

[rafael]

I tested this in our dev environment. Created a keyspace with 8 shards and split to 16. The process was super smooth.

[sougou]

Approving by proxy of @jvaidya