qkc/slave 05: add MasterConn protocol layer

[iteyelmp]

Summary

Add MasterConn, the slave-side connection implementation for communication
with the cluster master.

This PR builds MasterConn on top of the baseConn connection layer introduced
previously and focuses on protocol compatibility and handler registration.
Business logic migration is intentionally deferred.

Implemented

MasterConn

• TCP connection between slave and master using 12-byte ClusterMetadata framing
• Master-Slave opcode and serializer registration
• RPC request/response handling through baseConn
• Master handler registration and dispatch framework
• Protocol-compatible stub handlers for migrated message interfaces

Handler Registration

• Register Master-Slave cluster opcodes
• Register corresponding serializers
• Register RPC handlers
• Register DESTROY_CLUSTER_PEER_CONNECTION_COMMAND non-RPC handler
• Provide placeholder responses for unported business handlers

Testing

Unit tests validate:

• RPC handler dispatch
• request/response lifecycle
• metadata framing behavior
• connection lifecycle handling

Not in Scope

This PR does not migrate cluster business logic.