This commit addresses a panic error that occurred after the introduction
of a concurrency map in the pubsub by modifying the deletion process of
subscriptions. Now, instead of causing a panic, subs are deleted within
the callback to ensure thread safety and prevent race conditions.

)

This commit modifies the version check logic for operations, as the
previous SDK created operations that utilized an empty map for the
version vector instead of nil. 

With the introduction of version vector from v0.5.7, we now need to
differentiate between the Concurrent-Causal relationship while still
supporting the older operations generated by versions prior to v0.5.6.
The adjustment ensures that the logic appropriately processes an empty
map for version vector.

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Co-authored-by: Youngteac Hong <[email protected]>