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_core/cryptonote_core.cpp

@@ -163,6 +163,11 @@ namespace cryptonote
   , "Relay blocks as normal blocks"
   , false
   };
+  static const command_line::arg_descriptor<bool> arg_pad_transactions  = {
+    "pad-transactions"
+  , "Pad relayed transactions to help defend against traffic volume analysis"
+  , false
+  };
   static const command_line::arg_descriptor<size_t> arg_max_txpool_weight  = {
     "max-txpool-weight"
   , "Set maximum txpool weight in bytes."
@@ -188,7 +193,8 @@ namespace cryptonote
               m_disable_dns_checkpoints(false),
               m_update_download(0),
               m_nettype(UNDEFINED),
-              m_update_available(false)
+              m_update_available(false),
+              m_pad_transactions(false)
   {
     m_checkpoints_updating.clear();
     set_cryptonote_protocol(pprotocol);
@@ -282,6 +288,7 @@ namespace cryptonote
     command_line::add_arg(desc, arg_offline);
     command_line::add_arg(desc, arg_disable_dns_checkpoints);
     command_line::add_arg(desc, arg_max_txpool_weight);
+    command_line::add_arg(desc, arg_pad_transactions);
     command_line::add_arg(desc, arg_block_notify);
 
     miner::init_options(desc);
@@ -320,6 +327,7 @@ namespace cryptonote
     set_enforce_dns_checkpoints(command_line::get_arg(vm, arg_dns_checkpoints));
     test_drop_download_height(command_line::get_arg(vm, arg_test_drop_download_height));
     m_fluffy_blocks_enabled = !get_arg(vm, arg_no_fluffy_blocks);
+    m_pad_transactions = get_arg(vm, arg_pad_transactions);
     m_offline = get_arg(vm, arg_offline);
     m_disable_dns_checkpoints = get_arg(vm, arg_disable_dns_checkpoints);
     if (!command_line::is_arg_defaulted(vm, arg_fluffy_blocks))