[Consensus] Fix concurrent access to IoTConsensus configuration

[jt2594838]

Description

Concurrent configuration storage

• Replace the mutable TreeSet used by IoTConsensusServerImpl with a concurrent set backed by ConcurrentHashMap.newKeySet().
• Accept the initial peer collection through Collection<Peer> and copy it into the internal concurrent set so callers cannot supply a non-thread-safe implementation.

Deterministic configuration snapshots

• Sort the copied peer list in getConfiguration() to preserve the deterministic iteration order previously provided by TreeSet.

Concurrency coverage

• Add a unit test with four concurrent writers and four concurrent readers.
• Writers add and remove distinct peers while readers continuously obtain configuration snapshots.
• Verify that snapshots stay duplicate-free and sorted, and that the final membership is exact.

Verification

• mvn test -pl iotdb-core/consensus -Dtest=IoTConsensusServerImplTest,ReplicateTest,StabilityTest
• 7 tests passed; Checkstyle and Spotless passed.

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This PR has:

• been self-reviewed.
  • concurrent read
  • concurrent write
  • concurrent read and write
• added comments explaining the test scenario and expected result.
• added unit tests to cover the concurrent access path.

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Key changed/added classes

• IoTConsensusServerImpl
• IoTConsensus
• IoTConsensusServerImplTest