box: add auth_history and last_modified fields to _user space

See the doc bot request for the description of the new fields.

Note that we only store the value of the 'last_modified' field
in struct user_def, because 'auth_history' will be used only in
Lua code.

Needed for https://github.com/tarantool/tarantool-ee/issues/298
Needed for https://github.com/tarantool/tarantool-ee/issues/299

NO_CHANGELOG=no user-visible effects in CE; will be added to EE

@TarantoolBot document
Title: Document auth_history and last_modified _user space fields

Field name: auth_history. Field no: 6. Type: array.
Description: The field stores an array of previous authentication data:
when a user password is changed, the last value of the 'auth' field is
appended to 'auth_history'.  The length of the history is configured by
the `box.cfg.password_history_length` option, which is available only
in Tarantool EE, where it's used to prevent users from reusing old
passwords. In Tarantool CE, the array is always empty.

Field name: last_modified. Field no: 7. Type: unsigned.
Description: The field stores the timestamp (seconds since Unix epoch)
of the last user password update. It's never used in Tarantool CE.
In Tarantool EE, it's used to disable users that haven't changed the
password for more than `box.cfg.password_lifetime_days`.

`box.schema.upgrade()` sets the new field values to an empty array
and 0 for users that haven't updated them yet.