configuration option whether to require secure rekeying (on by default)
check rekeying data only if we require secure rekeying in client
respect configuration in respect to secure rekeying - if not required, don't check
rework rekeying; semantics being:
- if use_rekeying is false, fail rekeying requests on both sides (server:renegotiate ; client:get_rekeying_data)
- if we received some data from the other side
  -> check whether we have rekeying data; require equality (or empty if None)
- if we received ciphersuite and have no rekeying data (client_hello only, thus in server), return
- if not required, return
- if required, fail
change API of max/min_protocol_version and supported_protocol_versions;
use tls_version list instead of config
simplify further: move supported_protocol_version to handshake_common and
max/min_protocol_version to core, and adjust clients of these
signal a WARNING level alert in-band when configuration requests to not renegotiate, but server asked us
protocol_versions is now a tuple (high, low), specifying the protocol version range

simplifies quite some code
Merge pull request #100 from mirleft/config-secure-rekeying

Config secure rekeying