message: MutableStorage::commit() returns error on subseqt calls
diff: @@ -137,7 +137,10 @@ namespace iroha {
  expected::Result<MutableStorage::CommitResult, std::string>
  MutableStorageImpl::commit() && {
  + if (committed) {
  assert(not committed);
  + return "Tried to commit mutable storage twice.";
  + }
  assert(ledger_state_);
  try {
  sql_ << "COMMIT";
  