box: add an ability to disable CK constraints

Now it is possible to disable and enable ck constraints in LUA.
This option is persistent. All ck constraints are constructed
in enabled state when Tarantool is configured. This ability
may be usefulwhen processed data is verified and constraints
validation is not required. For instance, during casual recovery
process there's no need to provide any checks since data is
assumed to be consistent.

Persisting is_enabled flag is an important feature.
If the option is not stored the following scenario is
possible:
- the option is turned off
- data is changed so that a constraint is violated
- the system is restarted while the option state is lost
- there is no way (even in theory) to discover it and find that
  data is incorrect.

Part of #4244