Refactor `Replicate` message

[xirc]

RaftActor can relatively easily recognize whether the given Replicate message is for internal use or an entity.

Changes:

• Change Replicate to sealed trait
• An instance of Replicate is one of the following:
  • ReplicateForInternal: Used for internal use (RaftActor sends this message to itself)
  • ReplicateForEntity: Used for an entity (An entity sends this message to its RaftActor)

Considerations:

• This change doesn't affect serialization since Replicate is not a remote message.
• This change doesn't require rewriting the existing code.

Since we have a plan to change Leader's Replicate message handling, this PR is a preparation for future change.