net_processing: store transactions for private broadcast in PeerManager

Extend `PeerManager` with a transaction storage and a new method
`InitiateTxBroadcastPrivate()` which:
* adds a transaction to that storage and
* calls `CConnman::PrivateBroadcast::NumToOpenAdd()` to open dedicated
  privacy connections that will pick an entry from the transaction
  storage and broadcast it.

src/node/transaction.cpp

@@ -139,6 +139,7 @@ TransactionError BroadcastTransaction(NodeContext& node,
         node.peerman->InitiateTxBroadcastToAll(txid, wtxid);
         break;
     case TxBroadcast::NO_MEMPOOL_PRIVATE_BROADCAST:
+        node.peerman->InitiateTxBroadcastPrivate(tx);
         break;
     }