[Java] feat: add empty-queue skip scheduling for SimpleConsumer long-polling

[crazywen]

Motivation

When using SimpleConsumer with long-polling, if message distribution across queues
is uneven (some queues empty, others with messages), the consumer gets blocked by
long-polling on empty queues, causing significant delay in receiving messages from
queues that actually have data.

This is especially problematic in production with multiple queues where message
traffic is bursty.

Modification

Enhanced SubscriptionLoadBalancer with empty-queue skip scheduling:

• After each receiveMessage call, the result (empty or not) is fed back to the
  SubscriptionLoadBalancer via markEmptyResult()/markNonEmptyResult()
• Queues that consecutively return empty are temporarily skipped in takeMessageQueue()
• Skip duration is proportional to consecutive empty count (capped at 1000 rounds)
• When a queue returns messages again, its empty counter resets immediately
• Uses AtomicIntegerFieldUpdater for thread-safe state updates
• Fully backward-compatible: no API changes, no configuration required

Result

• Queues with messages are polled more frequently
• Overall message reception latency reduced in uneven-distribution scenarios
• Zero overhead when all queues have messages (skip map stays empty)
• Added unit tests for SubscriptionLoadBalancer

Type

• Enhancement