[raft][gate 2] Limit the size of the voting set of nodes

[MariemBaccari]

Context

The Raft library distinguishes voter nodes who count towards quorum from learners who receive and apply the log without voting. Our design uses learner status in the following ways:

• Transient during a join: a node joining a pool starts as a learner to avoid widening the quorum before it can actually ack anything. When the node catches up, it can be promoted to a voter status.
• Standing as a scalability measure: a node remains a learner and is never promoted to voter. This decouples the voting set from overall pool membership and mitigates the impact of a large pool's quorum size. We can therefore add nodes to the pool without growing the quorum.

Scope

• A node is added as a learner by default
• Define and implement the condition under which a learner is considered caught up and eligible for promotion
• Promotion / demotion support (per-group)
• dssctl must distinguish the cases defined above
• Document the recommended workflow

Out-of-Scope

• The add/remove membership change mechanisms (tracked in [raft][gate 2] Pool membership management #1719)

Expected result

• A newly added node starts as a learner and is visible as such via a dssctl listing command
• The pool defines and applies a concrete catch-up condition to know when a learner may be promoted
• A node can be added as a standing learner that is never auto-promoted
• A node can be promoted or demoted via dssctl
• Procedure is clearly documented with the trade-offs of voter / learner statuses clearly outlined