Refactor struct intertwined in cluster package to have a clear consistent naming and usage.

[reyreaud-l]

The cluster package currently implements the following structs, with the following tree hierarchy:

• cluster/ struct Service (Handles instantiation of RAFT, internal RPC service start and RPC client to leader)
  • Embeds cluster/service.go struct Service
    • Uses cluster/store.go struct Store
      • Uses raft.Raft from Hashicorp's RAFT to implement RAFT using the internal RPC client instantiated by the top-level struct (receives an interface) to talk to other nodes.
      • Uses *localDB from cluster/db.go as the underlying schema implementation. This layer implements the parsing of requests from Apply and then forwards them to the underlying schema. It also updates the data DB if needed.
        • Uses *schema from cluster/schema.go as the underlying schema implementation
          • TODO
        • Uses Indexer interface that is implemented by usecases/executor as the underlying data DB Implementation
      • Returns a retrySchema from cluster/retry_schema.go using localDB.schema when asked to provide as schema with SchemaReader
  • Uses transport/client.go struct Client (Internal RPC client for RAFT operation Join/Remove/etc...)
    • Implements the Query/Execute endpoint that are used to read/write from/to the leader
  • Uses transport/service.go struct Service (Internal RPC service for RAFT operation Join/Remove/etc...)
    • Implements the Query/Execute endpoint that are used to read/write from/to the leader