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.

tests/core_proxy/core_proxy.h

@@ -104,5 +104,6 @@ namespace tests
     cryptonote::difficulty_type get_block_cumulative_difficulty(uint64_t height) const { return 0; }
     bool fluffy_blocks_enabled() const { return false; }
     uint64_t prevalidate_block_hashes(uint64_t height, const std::vector<crypto::hash> &hashes) { return 0; }
+    bool pad_transactions() const { return false; }
   };
 }