raft: repurpose unused Node interface

[pav-kv]

We only use RawNode interface to raft, and don't use the Node which is a wrapper around RawNode with goroutines and channels that runs the node. In CRDB, we do all that in the raft scheduler.

We could outright remove the Node interface, but maybe we should leave if for additional testing coverage, or migrate the tests to use RawNode. The Node tests tend to be flaky (e.g. #121745) because of timing/scheduling: e.g. a test may wait for a leader and then assume that the leader is stable, but timing flakes can result in a leader change in the middle of the test.

A better approach to testing would be to have randomized, but deterministic tests based on RawNode. Then all the flakes would be preventable and/or reproducible.

We probably can repurpose Node to be a test-only type, with added benefits of randomized testing. We could improve reproducibility of these runs, by tracing all Step/Ready calls and message sends/drops done by the Nodes. The trace would be convertible to a list of datadriven commands that TestInteraction operates with, so any test run could be then reliably reproduced with RawNodes.

Jira issue: CRDB-37718

Epic CRDB-39898

[blathers-crl]

cc @cockroachdb/replication