chore(dataobj): Use kgo Balancer for dataobj-consumer

[benclive]

What this PR does / why we need it:

• Switches to the existing CooperativeSticky load balancing strategy for multi-tenant streams. The custom strategy is no longer required because we don't have to account for active/inactive partitions.
• Also used a buffer pool for flushing, which avoids needing to permanently allocate a large buffer that we only use once every few minutes
  • The pool is shared between all producers which reduces total memory usage but may still spike if many objects need to be flushed at once. We could probably improve this via some coordination / flush workers but thats for another day.