Merge bitcoin/bitcoin#21845: net processing: Don't require locking cs_main before calling RelayTransactions()

39e19713cd [net processing] Add internal _RelayTransactions() (John Newbery)

Pull request description:

  As part of the general effort to reduce cs_main usage in net_processing, this removes the need to be holding `cs_main` when calling `RelayTransactions()` from outside net_processing. Internally, we lock `cs_main` and call an internal `_RelayTransactions()` function that _does_ require `cs_main`.

ACKs for top commit:
  MarcoFalke:
    re-unsigned-code-review ACK 39e19713cd
  promag:
    Code review ACK 39e19713cd, just included sync.h since last review.
  ajtowns:
    ACK 39e19713cd

Tree-SHA512: dc08441233adfb8eaac501cf497cb4bad029eb723bd3fa8a3d8b7e49cc984c98859b95780ad15f5701d62ac745a8223beb0df405e3d49d95a8c86c8be17c9543

src/node/transaction.cpp

@@ -100,8 +100,6 @@ TransactionError BroadcastTransaction(NodeContext& node, const CTransactionRef t
         // the mempool tracks locally submitted transactions to make a
         // best-effort of initial broadcast
         node.mempool->AddUnbroadcastTx(hashTx);
-
-        LOCK(cs_main);
         node.peerman->RelayTransaction(hashTx, tx->GetWitnessHash());
     }