dcsctp: Merge ReconfigResponseSN/ReconfigRequestSN

Adding strong types went a little too far as these two types represent
the same sequence number. A "request sequence number" is a number, that
- when responded to - will be used as "response sequence number".

Having them separate added confusion and just a lot of type-casting.

Bug: webrtc:12614
Change-Id: I4636ea8f2252023a2d5a9b7033763e1978b1812e
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/214130
Commit-Queue: Victor Boivie <boivie@webrtc.org>
Reviewed-by: Florent Castelli <orphis@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33789}

net/dcsctp/packet/parameter/parameter_test.cc

@@ -29,7 +29,7 @@ TEST(ParameterTest, SerializeDeserializeParameter) {
   Parameters parameters =
       Parameters::Builder()
           .Add(OutgoingSSNResetRequestParameter(ReconfigRequestSN(123),
-                                                ReconfigResponseSN(456),
+                                                ReconfigRequestSN(456),
                                                 TSN(789), {StreamID(42)}))
           .Build();