Use "/path" RPC routes to call plugin-defined endpoints.
Use ".proc" RPC routes to call builtin picodata endpoints (for example
.proc_instance_info)

- use RouteBuilder to register rpc endpoints (input and ouptut is raw
  bytes for now)
- use RequestBuilder to send RPC requests to other instances which are
  running a specified plugin.service (guarded by version)
    - supported routing by
        - explicit instance_id
        - replicaset_id
        - bucket_id
        - any available instance with given service
- use context::Context to extract information about request or add
  custom fields at runtime (middleware not yet implemented)

The problem is MsgTimeoutNow always results in a new election, because
it explicitly ignores the pre_vote step. This results in integration
test flakiness whenever a follower calls promote_or_fail while the
leader's raft log is longer.

We've cherry-picked some fixes but didnt cut a release yet. Prepare
changelog.