protocol: option to pad transaction relay to the next kB

To help protect one's privacy from traffic volume analysis
for people using Tor or I2P. This will really fly once we
relay txes on a timer rather than on demand, though.

Off by default for now since it's wasteful and doesn't bring
anything until I2P's in.

src/cryptonote_protocol/cryptonote_protocol_defs.h

@@ -146,9 +146,11 @@ namespace cryptonote
     struct request
     {
       std::vector<blobdata>   txs;
+      std::string _; // padding
 
       BEGIN_KV_SERIALIZE_MAP()
         KV_SERIALIZE(txs)
+        KV_SERIALIZE(_)
       END_KV_SERIALIZE_MAP()
     };
   };