cs: improve ExchangeResponseMessage

* The synchronized time is set but never read. Remove it.
* Rename the remaining time to unsynchronized_time.
Consistently use std::strings to hold ports...

...except for devices. We'll change those later.
Remove FIXMEs related to sending times as strings

This turns out to be an easy portable solution. (E.g. we don't even know
that std::time_t is an integral type, let alone its size.)
CConnection: store an arbitrary protocol

Still set to CProtocolSR in the constructor. There must be a better way
to handle this.