encapsulates turbine peers computations of broadcast & retransmit stages (#18238)

Broadcast stage and retransmit stage should arrange nodes on turbine
broadcast tree in exactly same order. Additionally any changes to this
ordering (e.g. updating how unstaked nodes are handled) requires feature
gating to keep the cluster in sync.

Current implementation is scattered out over several public methods and
exposes too much of implementation details (e.g. usize indices into
peers vector) which makes code changes and checking for feature
activations more difficult.

This commit encapsulates turbine peer computations into a new struct,
and only exposes two public methods, get_broadcast_peer and
get_retransmit_peers, for call-sites.

core/src/lib.rs

@@ -12,6 +12,7 @@ pub mod banking_stage;
 pub mod broadcast_stage;
 pub mod cache_block_meta_service;
 pub mod cluster_info_vote_listener;
+pub mod cluster_nodes;
 pub mod cluster_slot_state_verifier;
 pub mod cluster_slots;
 pub mod cluster_slots_service;