Merge bitcoin/bitcoin#22340: p2p: Use legacy relaying to download blocks in blocks-only mode

18c5b23a0f [test] Test that -blocksonly nodes still serve compact blocks. (Niklas Gögge)
a79ad65fc2 [test] Test that getdata(CMPCT) is still sent on regular low bandwidth connections. (Niklas Gögge)
5e231c116b [test] Test that -blocksonly nodes do not send getdata(CMPCT) on a low bandwidth connection. (Niklas Gögge)
5bf6587457 [test] Test that -blocksonly nodes do not request high bandwidth mode. (Niklas Gögge)
0dc8bf5b92 [net processing] Dont request compact blocks in blocks-only mode (Niklas Gögge)

Pull request description:

  A blocks-only node does not participate in transaction relay to reduce its own bandwidth usage and therefore does not have a mempool. The use of compact blocks is not beneficial to such a node since it will always have to download full blocks.

  In both high- and low-bandwidth relaying the `cmpctblock` message is sent. This represent a bandwidth overhead for blocks-only nodes because the `cmpctblock` message is several times larger in the average case than the equivalent `headers` or `inv` announcement.

  ![compact blocks](https://raw.githubusercontent.com/bitcoin/bips/master/bip-0152/protocol-flow.png)

  >**Example:**
  >A block with 2000 txs results in a `cmpctblock` with 2000*6 bytes in short ids. This is several times larger than the equivalent 82 bytes for a `headers` message or 37 bytes for an `inv`.

  ## Approach

  This PR makes blocks-only nodes always use the legacy relaying to download new blocks.
  It does so by making blocks-only nodes never initiate a high-bandwidth block relay connection by disabling the sending of `sendcmpct(1)`. Additionally a blocks-only node will never request a compact block using `getdata(CMPCT)`.

  A blocks-only node will continue to serve compact blocks to its peers in both high- and low-bandwidth mode.

ACKs for top commit:
  naumenkogs:
    ACK 18c5b23a0f
  rajarshimaitra:
    tACK 18c5b23a0f
  jnewbery:
    reACK 18c5b23a0f
  theStack:
    re-ACK 18c5b23a0f 🥛

Tree-SHA512: 0c78804aa397513d41f97fe314efb815efcd852d452dd903df9d4749280cd3faaa010fa9b51d7d5168b8a77e08c8ab0a491ecdbdb3202f2e9cd5137cddc74624

test/functional/test_runner.py

@@ -98,6 +98,7 @@ BASE_SCRIPTS = [
     'rpc_fundrawtransaction.py --legacy-wallet',
     'rpc_fundrawtransaction.py --descriptors',
     'p2p_compactblocks.py',
+    'p2p_compactblocks_blocksonly.py',
     'feature_segwit.py --legacy-wallet',
     # vv Tests less than 2m vv
     'wallet_basic.py --legacy-wallet',