Add 'ignore_undefined_slaves' to synchronous ModbusSerialServer

The default behaviour of pymodbus is to return an error to all
messages addressed to a slave unit that is not defined in the
given server context. An option exists for responding to all
slave IDs with a single context, but not to ignore messages to
any undefined slave ID.

This creates a problem when pymodbus is sharing a bus with other
units, as it sends error messages clobbering the proper responses
from the rightful addressees.

This patch adds an ignore_undefined_slaves boolean parameter to
the synchronous ModbusSerialServer. For compatibility with older
programs, default has been left to False.

For now, the option is only on the synchronous serial server, as
it's the one I needed. If the patch is accepted, I'll be happy to
add this option also to the IP based servers and all async servers.