Address multiple issues with Buffer and Channel

- StreamDataMessage now packed by struct rather than umsgpack for a more predictable size
- Added protected variable on LocalInterface to allow tests to simulate a low bandwidth connection
- Retry timer now has exponential backoff and a more sane starting value
- Link proves packet _before_ sending contents to Channel; this should help prevent spurious retries especially on half-duplex links
- Prevent Transport packet filter from filtering out duplicate packets for Channel; handle duplicates in Channel to ensure the packet is reproven (in case the original proof packet was lost)
- Fix up other tests broken by these changes

RNS/Link.py

@@ -809,9 +809,9 @@ class Link:
                         if not self._channel:
                             RNS.log(f"Channel data received without open channel", RNS.LOG_DEBUG)
                         else:
+                            packet.prove()
                             plaintext = self.decrypt(packet.data)
                             self._channel._receive(plaintext)
-                            packet.prove()
 
                 elif packet.packet_type == RNS.Packet.PROOF:
                     if packet.context == RNS.Packet.RESOURCE_PRF: