feat: Key watching

[Migorithm]

Description

• User "watches" a key, subscribing to changes
• When the key is updated, the server notified all watchers

Note that this feature is different than PUB/SUB as it actually stores the data.
This is key-specific not user-defined channel specific.

Acceptance Criteria

• Keys are shared across cluster. Then each node should manage watches for the keys it owns
• Key migration(resharding) must move watch state as well

Notes

No response