network and time fixes

programs/umnp-rhtp.py

umnp-daq.py

umnp/microcontroller/communication/serial_uart.py

umnp/microcontroller/devices/network/ethernet_w5500.py

umnp/microcontroller/measurementdevice.py

umnp/microcontroller/tasks/periodictask.py

umnp/protocol/common/timestamp.py

umnp/protocol/compat/datetime.py

umnp/protocol/compat/time.py

umnp/protocol/constants.py

umnp/protocol/message.py

umnp/protocol/message_header.py

umnp/protocol/umnp_time.py