Fix concurrent modification error when closing socket

[stnguyen90]

What does this PR do?

Since subscription.close() modifies _subscriptions, we need to create a separate list to avoid concurrent modification error.

Closes appwrite/sdk-for-flutter#200

Test Plan

Manually tested and confirmed subscribing and unsubscribing still works:

Related PRs and Issues

• 🐛 Bug Report: RealTime subscribe throws Concurrent modification during iteration: _Map len:0 sdk-for-flutter#200

Have you read the Contributing Guidelines on issues?

Yes

[lohanidamodar]

Wait will this also notify the subscriber when we make copy of the list do the subscriber still get notification?

[stnguyen90]

Wait will this also notify the subscriber when we make copy of the list do the subscriber still get notification?

@lohanidamodar, dart doesn't make a copy on assignment; they all reference the same objects.