message: Ref Cleanup a bit
  The contract handlers that need the database get a reference to it in
  the context. I don't need to pass another db reference to them. Fix
  this.
diff: @@ -56,12 +56,9 @@ void native_system_contract_plugin::plugin_shutdown() {
  void native_system_contract_plugin::install(database& db) {
  #define SET_HANDLERS(name) \
  - db.set_validate_handler("sys", "sys", #name, \
  - [&db](chain::message_validate_context& c) mutable { name ## _validate(c); }); \
  - db.set_precondition_validate_handler("sys", "sys", #name, \
  - [&db](chain::precondition_validate_context& c) mutable { name ## _validate_preconditions(c); }); \
  - db.set_apply_handler("sys", "sys", #name, \
  - [&db](chain::apply_context& c) mutable { name ## _apply(c); })
  + db.set_validate_handler("sys", "sys", #name, &name ## _validate); \
  + db.set_precondition_validate_handler("sys", "sys", #name, &name ## _validate_preconditions); \
  + db.set_apply_handler("sys", "sys", #name, &name ## _apply)
  SET_HANDLERS(Transfer);
  }
  