Issue with simultaneous Attachment to a new document leading to element coexistence without GC mapping

[raararaara]

What happened:
When two clients simultaneously attach to a new document using Attach with InitialRoot, a scenario occurs where one element is not added to gcElementPairMap, resulting in two elements existing concurrently in root.ElementsMapByCreatedAt without the expected mapping in root.gcElementPairMap.

What you expected to happen:
Only one instance of the alive element should exist in Root.ElementsMapByCreatedAt, with proper mapping in root.gcElementPairMap.

How to reproduce it (as minimally and precisely as possible):

1. Use two clients to simultaneously attach to a new document with Attach with InitialRoot. (codemirror example in yorkie-js-sdk, etc)
2. Observe the root.ElementsMapByCreatedAt and root.gcElementPairMap post-attachment.

Anything else we need to know?:
This appears to be a concurrency handling issue, and further investigation is necessary to determine the root cause.

Environment:

• Operating system:
• Browser and version:
• Yorkie version: v0.6.14
• Yorkie JS SDK version: v0.6.14