Reconnect without timeout after downgrade

The implemented behavior in the rtrlib is now the following

- If the cache sends a PDU with different version then currently active and it´s
  the first PDU we´ve received the client downgrades on the fly
  (Without terminating the connection).

- If the PDU isn't the first one we´ve received, the client terminates the connection
  and waits for 'Retry Interval' seconds. I think this is the beste solution, because it´s
  very likely that something is wrong in the cache implementation, if it´s changing the
  version after we have already exchanged PDUs.

- If we receive an 'unsupported version' error PDU we reconnect immediately with a lower
  protocol version.