net: Make AddrFetch connections to fixed seeds

In order to reduce the load on fixed seeds that will receive
potentially many peers requiring IBD, just do an
AddrFetch (so that we disconnect them after receiving
addresses from them).

Do that with up to 10 fixed seeds for diversity. The fixed seeds
continue to be added to AddrMan afterwards, which at this point should
contain multiple other addresses received from the AddrFetch peers.

src/net.cpp

@@ -2320,6 +2320,28 @@ void CConnman::ProcessFixedSeeds(std::chrono::microseconds start)
                 seed_addrs.erase(std::remove_if(seed_addrs.begin(), seed_addrs.end(),
                                                 [&fixed_seed_networks](const CAddress& addr) { return fixed_seed_networks.count(addr.GetNetwork()) == 0; }),
                                  seed_addrs.end());
+                Shuffle(seed_addrs.begin(), seed_addrs.end(), FastRandomContext());
+
+                // Make AddrFetch connections to 10 fixed seeds first. This reduces the
+                // load on the fixed seeds that would otherwise be serving blocks for
+                // many new peers requiring IBD. Try this with multiple fixed seeds for
+                // a better diversity of received addrs and because some may be offline.
+                const std::chrono::minutes wait_time{2};
+                LogPrintf("Initiating AddrFetch connections to fixed seeds. This might take up to %d minutes.\n", wait_time.count());
+                for (size_t addr_pos = 0; addr_pos < 10; ++addr_pos) {
+                    if (addr_pos >= seed_addrs.size()) {
+                        break;
+                    }
+                    AddAddrFetch(seed_addrs.at(addr_pos).ToStringAddr());
+                }
+                // Give AddrFetch peers some time to provide us with addresses
+                // before adding the fixed seeds to AddrMan
+                if (!interruptNet.sleep_for(wait_time)) {
+                    return;
+                }
+                // The fixed seeds queried in the previous steps might have been offline,
+                // failed to send us any addresses or sent us fake ones. As a fallback for this,
+                // we now add all reachable fixed seeds to AddrMan.
                 CNetAddr local;
                 local.SetInternal("fixedseeds");
                 addrman.Add(seed_addrs, local);