Various small improvements and code cleanup

[popduke]

1. Improved HLC impl and documentation
2. Improved the ReplicaCntBalancer to better handle an edge case
3. Improved session memory usage estimation
4. Added contextual tags to heuristic compactor's metrics in basekv-local-engine
5. Enabled pin data for kv iterator to improve memory consumption in some cases

[coveralls]

Pull Request Test Coverage Report for Build 16618521738

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 283 unchanged lines in 13 files lost coverage.
• Overall coverage decreased (-0.006%) to 63.066%

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Files with Coverage Reduction	New Missed Lines	%
org/apache/bifromq/basekv/raft/RaftNodeStateLeader.java	1	85.23%
org/apache/bifromq/mqtt/handler/v3/MQTT3PersistentSessionHandler.java	1	88.89%
org/apache/bifromq/basecrdt/service/CRDTCluster.java	2	81.91%
org/apache/bifromq/basekv/raft/RaftNodeStateFollower.java	2	82.11%
org/apache/bifromq/basekv/store/stats/StatsCollector.java	2	78.95%
org/apache/bifromq/dist/trie/TopicFilterIterator.java	2	70.99%
org/apache/bifromq/basekv/localengine/rocksdb/RocksDBKVEngineIterator.java	7	78.57%
org/apache/bifromq/basekv/balance/impl/ReplicaCntBalancer.java	13	81.02%
org/apache/bifromq/basecluster/memberlist/AutoDropper.java	14	75.0%
org/apache/bifromq/basehlc/HLC.java	15	61.73%

Totals
Change from base Build 16591047944:	-0.006%
Covered Lines:	23589
Relevant Lines:	36106

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💛 - Coveralls