Switch to hash-based dictionary for effective subscribers collection

[crazyrokr]

Benchmark	Mode	Cnt	Score	Error	Units
optimizedImplementation	thrpt	20	3651.882	± 4.731	ops/ms
optimizedImplementation:gc.alloc.rate	thrpt	20	0.005	± 0.002	MB/sec
optimizedImplementation:gc.alloc.rate.norm	thrpt	20	0.001	± 0.001	B/op
optimizedImplementation:gc.count	thrpt	20	≈ 0		counts
originalImplementation	thrpt	20	1511.932	± 4.273	ops/ms
originalImplementation:gc.alloc.rate	thrpt	20	0.005	± 0.002	MB/sec
originalImplementation:gc.alloc.rate.norm	thrpt	20	0.003	± 0.001	B/op
originalImplementation:gc.count	thrpt	20	≈ 0		counts

[github-actions]

Test Coverage Report

Overall Project	85.33% -0.04%	🍏
Files changed	98.14%	🍏

File	Coverage
ConcurrentSubscriberTree.java	100%	🍏
OptimizedSubscriberNode.java	98.98% -1.02%	🍏
InMemorySubscriptionService.java	96.59%	🍏
MqttBrokerSpringConfig.java	89.79%	🍏
SubscriberTreeBase.java	69.96%	🍏
SubscriberNode.java	0% -1.22%	❌