msgr2 supports two connection modes: crc (plain) and secure (on-wire
encryption).  Connection mode is picked by server based on input from
client.

Introduce ms_mode option:

  ms_mode=legacy        - msgr1 (default)
  ms_mode=crc           - crc mode, if denied fail
  ms_mode=secure        - secure mode, if denied fail
  ms_mode=prefer-crc    - crc mode, if denied agree to secure mode
  ms_mode=prefer-secure - secure mode, if denied agree to crc mode

ms_mode affects all connections, we don't separate connections to mons
like it's done in userspace with ms_client_mode vs ms_mon_client_mode.

For now the default is legacy, to be flipped to prefer-crc after some
time.
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>