Factor out the cache reset logic into rtr_reset_cache() and use it

Factor out the cache reset logic into rtr_reset_cache() and use it
consitently in all places where the cache should be reset.

Additionally adjust the FSM to handle a connection close event in the
protocol negotiation phase as a protocol mismatch. Some caches fail
to send an error PDU and this allows them to work. A drawback is that
if the connection is closed because of a different reason the system
will fall back to a lower then necessary version. The RFC and draft
mandates this behaviour.

OK tb@