Keep the previous stream when executing AttachDocument or DetachDocument

[hackerwins]

What would you like to be added:

Keep the previous stream when executing AttachDocument or DetachDocument

Since gRPC web does not provide a bi-directional stream, it is difficult to pass subscription information for new documents directly from WatchDocument.

When executing AttachDocument or DetachDocument, the document keys of subscription established in WatchDocuments must be updated.

https://github.com/yorkie-team/yorkie/blob/105a9a832ba98fe62f1be0708146fae25fd9eaed/yorkie/rpc/yorkie_server.go#L335-L339

Why is this needed:

• When attaching multiple documents to one client, unnecessary reconnection occurs.

[hackerwins]

Related to https://github.com/yorkie-team/yorkie-team.github.io/issues/21#issuecomment-996992572

[blurfx]

I'm interested in this issue. Is it easy to reproduce this problem?,

[hackerwins]

Yes, it will be easy to reproduce. :)

[hackerwins]

With support for shading-based clusters, a single client now has multiple connections per document. Therefore, this issue could be closed.

https://github.com/yorkie-team/yorkie/commit/dcf4cfb6ff98173c6d02a0abed89cb847f8aaa1e