revert to use u instead of t for timestamp in RC for backwards compat reasons

docs/proto_v0.txt

@@ -139,7 +139,7 @@ router's full identity
   k: "<32 bytes public long term identity signing key>",
   n: "<optional max 32 bytes router nickname>",
   p: "<32 bytes public path encryption key>",   
-  t: time_signed_at_milliseconds_since_epoch_uint64,
+  u: time_signed_at_milliseconds_since_epoch_uint64,
   v: 0,
   x: [ Exit, Infos ],
   z: "<64 bytes signature using identity key>"

llarp/router_contact.cpp

@@ -116,7 +116,7 @@ namespace llarp
       return false;
 
     /* write last updated */
-    if(!bencode_write_bytestring(buf, "t", 1))
+    if(!bencode_write_bytestring(buf, "u", 1))
       return false;
     if(!bencode_write_uint64(buf, last_updated))
       return false;
@@ -179,10 +179,10 @@ namespace llarp
     if(!BEncodeMaybeReadDictEntry("p", enckey, read, key, buf))
       return false;
 
-    if(!BEncodeMaybeReadDictInt("v", version, read, key, buf))
+    if(!BEncodeMaybeReadDictInt("u", last_updated, read, key, buf))
       return false;
 
-    if(!BEncodeMaybeReadDictInt("t", last_updated, read, key, buf))
+    if(!BEncodeMaybeReadDictInt("v", version, read, key, buf))
       return false;
 
     if(!BEncodeMaybeReadDictList("x", exits, read, key, buf))