message: schedule write to send datagrams
  Summary: invoke updateWriteLooper to make sure a datagram can be sent when it's the only thing to send
diff: @@ -2700,10 +2700,10 @@ QuicTransportBase::setDatagramCallback(DatagramCallback* cb) {
  uint16_t QuicTransportBase::getDatagramSizeLimit() const {
  CHECK(conn_);
  - auto maxDatagramPacketSize = std::min<uint64_t>(
  + auto maxDatagramPacketSize = std::min<decltype(conn_->udpSendPacketLen)>(
  conn_->datagramState.maxWriteFrameSize, conn_->udpSendPacketLen);
  - CHECK(maxDatagramPacketSize >= kMaxDatagramPacketOverhead);
  - return maxDatagramPacketSize - kMaxDatagramPacketOverhead;
  + return std::max<decltype(maxDatagramPacketSize)>(
  + 0, maxDatagramPacketSize - kMaxDatagramPacketOverhead);
  }
  folly::Expected<folly::Unit, LocalErrorCode> QuicTransportBase::writeDatagram(
  @@ -2725,6 +2725,7 @@ folly::Expected<folly::Unit, LocalErrorCode> QuicTransportBase::writeDatagram(
  }
  }
  conn_->datagramState.writeBuffer.emplace_back(std::move(buf));
  + updateWriteLooper(true);
  return folly::unit;
  }
  