use a std::map instead of a std::unordered_map for holding message buffers by their sequentially increasing message id.

when retransmissions happen, they will be executed in a deterministic order by their delivery id.
this reduces the jitter from delayed message delivery on link layer as with an unordered map the order of iteration is "random",
so when we iterate all messages for retransmission we do it lowest id first (the messages queued first).

llarp/iwp/session.hpp

@@ -6,6 +6,7 @@
 #include <iwp/message_buffer.hpp>
 #include <net/ip_address.hpp>
 
+#include <map>
 #include <unordered_set>
 #include <deque>
 #include <queue>
@@ -185,8 +186,8 @@ namespace llarp
       void
       ResetRates();
 
-      std::unordered_map<uint64_t, InboundMessage> m_RXMsgs;
-      std::unordered_map<uint64_t, OutboundMessage> m_TXMsgs;
+      std::map<uint64_t, InboundMessage> m_RXMsgs;
+      std::map<uint64_t, OutboundMessage> m_TXMsgs;
 
       /// maps rxid to time recieved
       std::unordered_map<uint64_t, llarp_time_t> m_ReplayFilter;