Resolve N+1 issue in GetDocuments API response when snapshot is included

[kokodak]

What would you like to be added:

Currently, when the GetDocuments API endpoint is called and the response needs to include a snapshot, an N+1 issue occurs. This problem arises because packs.BuildDocumentForServerSeq() is called internally N times.

Within packs.BuildDocumentForServerSeq(), two database queries are executed, leading to the N+1 problem. To resolve this issue, the database query within this method should be converted to a bulk version.

If there is a need for a snapshot preview when displaying documents in CodePair, resolving this issue could result in performance improvements.

yorkie/server/documents/documents.go

Lines 136 to 144 in a4ce314

	if includeSnapshot {
	// TODO(hackerwins, kokodak): Resolve the N+1 problem.
	doc, err := packs.BuildDocumentForServerSeq(ctx, be, docInfo, docInfo.ServerSeq)
	if err != nil {
	return nil, err
	}
	snapshot = doc.Marshal()
	}

Why is this needed:

To address the N+1 problem in the GetDocuments API response to enhance performance.

[kokodak]

kokodak@e600891

The attached commit is the code for a bulk query implementation I've worked on before.

In the case of FindClosestSnapshotInfo() bulk, the required query is rather complicated, so I configured the query by configuring the Aggregate pipeline.
When sorting and grouping in the pipeline, it would be good to consider the DB Index applied to the Snapshot Collection.

For the bulk of FindChangesBetweenServerSeqs(), there is a high probability of memory related issues, as it is looking for a very large number of Changes if a Snapshot is missing, and we need to be aware of this issue.