Implement wal_mode = none option.

Implement blueprint
https://blueprints.launchpad.net/tarantool/+spec/wal-mode-none

Add wal_mode = none setting.
It's possible to chagne wal_mode without restart.

When wal_mode = none, we don't write to WAL.
It's still possible to take a snapshot.

Replication and hot standby are currently not
operational in this mode.