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>
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